Can a domain block my emails without a bounce message?

Yes, it's possible. Sometimes, a domain might silently drop or quarantine your emails without sending a bounce message, especially if their filters are very aggressive or your email is caught in a spam trap. This is often referred to as a soft block . In these cases, you'll need to rely on indirect indicators like a consistent lack of response or confirmation from the recipient that they aren't receiving your emails.

What do 550 errors mean for email delivery?

SMTP error 550 is a common hard bounce error indicating a permanent failure. It typically means the recipient's email server actively rejected your message. This can be due to the recipient's address not existing, your sending domain or IP being on their internal blocklist, or your email being flagged as spam by their system. The specific message accompanying the 550 code usually provides more detail.

How often should I check blocklists if I only send to a few recipients?

Even when sending to only a few recipients, it's wise to periodically check public blocklists (or blacklists) to ensure your domain or IP hasn't been accidentally listed. While private or internal blocklists are more likely to affect specific, low-volume sends, a listing on a major public blocklist can severely impact all your email deliverability. A weekly or bi-weekly check is often sufficient for proactive monitoring.

Does my email content affect individual recipient blocking?

Yes, email content can absolutely affect individual recipient blocking. Aggressive spam filters can detect certain keywords, suspicious links, unusual formatting, or even specific attachments. If your email content is perceived as spammy or malicious, even by a single recipient's server, it can trigger a block against your email or domain, regardless of your sending volume. Ensuring your content is clean, relevant, and free of typical spam indicators is important.

How to identify if a domain has blocked you when emailing only a few recipients? - Troubleshooting - Email deliverability - Knowledge base

Trying to figure out why your emails aren't reaching a specific recipient, especially when you're only sending a few messages, can be incredibly frustrating. When you send emails in high volumes, identifying delivery issues is often straightforward because patterns emerge quickly. You see a spike in bounces to a particular domain, or an overall drop in inbox placement rates, and you know something is wrong.

However, with low volume, like emailing only one to four recipients at a given domain, those clear patterns disappear. Generic bounce responses for soft, block, or unknown bounces don't offer much insight. It becomes difficult to discern if the issue is a temporary glitch, a personal setting on the recipient's end, or if the entire domain has implemented a block against your sending infrastructure.

In these scenarios, a more targeted and nuanced approach is required. I'll walk you through the methods I use to pinpoint exactly why your emails aren't getting through when volume is low, helping you understand if you've been blocked and what steps you can take.

Deciphering bounce messages and SMTP codes

The most direct way to identify if a domain has blocked your emails is by carefully examining the bounce messages you receive. When an email is rejected by the recipient's mail server, it typically sends back an SMTP (Simple Mail Transfer Protocol) bounce code and a corresponding message. These messages are often the clearest indication of what went wrong.

Look specifically for 5xx series error codes, which indicate a permanent failure (a hard bounce). A common one is 550 which signifies a non-existent user, mailbox unavailable, or a general rejection due to policy. Sometimes, the accompanying message will explicitly state Address Rejected, Blocked by recipient policy, or even mention a specific blocklist or spam filter. Understanding what SMTP error 550 means is a critical first step.

Common SMTP Bounce Messagestext

550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in local recipient table
550 5.7.1 Service unavailable; Client host [XXX.XXX.XXX.XXX] blocked using Blocklist.COM
550 5.7.1 Message rejected as spam by Content Filtering

Even with a large volume of logs (thousands of bounces across thousands of domains), you can filter down to the specific recipient domain that is causing issues. Most email service providers (ESPs) or mail servers provide detailed bounce logs. You can export these logs and then use spreadsheet software to sort and filter by the recipient's domain and the SMTP bounce reason. This allows you to quickly isolate the handful of bounces from the domain in question and analyze their specific error messages, giving you a very clear picture of why they were rejected.

Typical reasons for 550 address rejected

Non-existent address: The email address you are sending to does not exist at the recipient's domain.
Recipient's blocklist: The recipient's email server (or the recipient personally) has added your sending address or domain to a local blocklist (or blacklist).
Spam detection: The recipient's server flagged your email as spam due to content, reputation, or other factors, resulting in a direct rejection rather than delivery to spam folder.
Policy violations: The email violates a specific security policy enforced by the recipient's domain.

Looking for indirect signs of a block

Sometimes, direct bounce messages aren't entirely conclusive, or they might not be returned at all in cases of subtle soft blocks. A soft block means your email appears to be delivered, but it never reaches the inbox (e.g., it's silently dropped or sent directly to spam without notification). This is especially challenging with low volume. As noted on The Email Marketers blog, many soft-blocked messages will appear as delivered.

One strategy is to perform a strategic test. If you have an alternative email address or a different sending domain with a good reputation, try sending a simple, plain-text email to the same recipient. If that email goes through without issues and your primary sending method continues to fail, it strongly suggests a block against your main domain or IP. Additionally, if you have an alternative communication channel (like phone or messaging app), you could consider reaching out to the recipient to ask if they've been receiving your emails, but only if appropriate for your relationship with them.

Clear bounce notices

Specific error codes: Look for hard bounce codes like 550 indicating permanent rejection.
Explicit block messages: Bounce messages explicitly stating Blocked by recipient policy.
Authentication failures: Messages indicating failed SPF, DKIM, or DMARC checks. For more details, consult our guide on what to do when emails are blocked.

Subtle signs

No replies: Consistent lack of response from a specific domain, even to important emails.
Recipient checks spam: The recipient confirms your emails are not reaching their inbox or are found in their spam folder.
Inconsistent delivery: Some emails might go through, while others to the same domain consistently fail without clear bounces.

When you encounter these more subtle signs, it’s crucial to cross-reference them with any available bounce data, no matter how generic it seems. Sometimes, a series of generic soft bounces followed by a complete lack of response can indicate that the recipient's domain has started to quietly block your mail, even if it's not explicitly stated.

Checking your domain and IP reputation

Even with only a few recipients, your sending domain or IP address might be listed on a private or localized blocklist (also known as a blacklist), or simply have a poor reputation with that specific recipient’s mail server. This isn't always about major public blacklists that affect bulk sending, but rather smaller, often internal, lists used by individual organizations. Internal reputation services can be particularly challenging to diagnose.

While public blacklists might not be the primary culprit for low-volume, specific blocks, regularly checking your domain's health against common blocklists is always a good practice. Many online tools allow you to check if your domain or IP is listed. Even if your domain isn't on a major public blocklist, its overall sending reputation might be low, triggering filters at stricter domains. According to Salesforce Ben, there are a number of domain and IP address blacklists (or blocklists) that can affect deliverability.

Type	Description	Impact on small sends
IP Blocklist	Lists problematic IP addresses, often due to spam or abuse.	Can block all emails from that IP to the affected domain, regardless of volume.
Domain Blocklist	Lists suspicious sending domains based on reputation.	Can block emails from your domain to specific mail servers. Barracuda is an example.
Recipient-specific Block	The recipient or their IT department explicitly blocks your address or domain.	Only impacts sends to that specific recipient or organization. Microsoft 365 allows domain blocking.

The challenge with these types of blocks is that they are often not publicly disclosed. While you can use a blocklist checker for major public lists, a private blocklist (or blacklist) will require deeper investigation using bounce messages and other indicators. This makes analyzing the bounce messages even more critical, as they may contain clues about the specific filter or system that's rejecting your mail.

Ensuring proper authentication and content

Even for low-volume sends, robust email authentication is non-negotiable. Missing or misconfigured SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), or DMARC (Domain-based Message Authentication, Reporting & Conformance) records can lead to emails being blocked, even if your content is perfect and your reputation is generally good. Recipient mail servers use these records to verify that your emails are legitimate and not spoofed.

Email authentication checklist

SPF record: Ensure your SPF record is correctly configured and includes all authorized sending IP addresses and services. An invalid SPF can cause rejections.
DKIM signature: Verify your DKIM signature is valid and hasn't expired. A missing or invalid DKIM can flag your email as suspicious.
DMARC policy: Implement a DMARC policy, even if it's set to p=none, to receive reports on your email authentication status. You can use our free DMARC record generator.
DMARC alignment: Ensure your SPF and DKIM pass the DMARC alignment checks, as even if they pass individually, misalignment can still cause issues.

Beyond technical authentication, the actual content of your email can also trigger recipient-specific blocks. Even a seemingly innocent email could be flagged by aggressive spam filters if it contains certain keywords, suspicious links (e.g., untrusted URLs), or attachments that the recipient's system deems risky. For example, GlockApps highlights that if an email is placed in the inbox for some, but not others, the problem may be related to a URL or domain used in the email content.

When troubleshooting low-volume blocks, review your email content for anything that might appear spammy, even if it's not. This includes excessive capitalization, too many images compared to text, or links that redirect multiple times. Maintaining a clean sending reputation and adhering to best practices is paramount, regardless of your sending volume. You can find more tips on preventing cold emails from harming your domain reputation in our other resources.

Views from the trenches

Best practices

Always keep a close eye on your bounce logs, especially for unique bounce messages.

Segment your bounce data by recipient domain to quickly spot specific blocking issues.

Ensure your SPF, DKIM, and DMARC records are always up-to-date and correctly configured.

Common pitfalls

Ignoring generic bounce messages, assuming they are always soft failures.

Failing to filter and analyze bounce logs for specific domains and SMTP responses.

Assuming low sending volume makes you immune to domain or IP-based blocklists.

Expert tips

Implement robust logging and analytics to easily slice and dice bounce data by domain and SMTP reason.

For low volume, calculate the email success rate as (# Attempts minus # Bounces) / # Attempts, sorted ascending, to highlight problem domains.

Regularly export and review raw bounce messages from your ESP, even for smaller datasets.

Expert view

Expert from Email Geeks says: The actual bounce message from the mailbox provider will tell you the reason for the block.

2024-07-06 - Email Geeks

Expert view

Expert from Email Geeks says: While statistics are useful for high volume, for small numbers of recipients, you have the luxury of hand-reviewing individual log entries.

2024-07-06 - Email Geeks

Proactive measures for email deliverability

Identifying if a domain has blocked your emails when sending to only a few recipients requires a detective-like approach. While large-scale analytics help with high-volume sending, low volume demands a keen eye on individual bounce messages and a systematic investigation of various factors.

By meticulously analyzing SMTP bounce codes and messages, looking for subtle indirect signs like lack of response, and consistently monitoring your domain and IP reputation, you can uncover why your messages aren't reaching their intended targets. Additionally, ensuring your email authentication (SPF, DKIM, DMARC) is flawless and your content is clean will significantly reduce the chances of encountering these frustrating blocks.

Proactive monitoring and a willingness to dig into the technical details are your best defense against unexpected email deliverability issues, ensuring your important messages always make it to the inbox.