Does a sending IP address need to accept incoming SMTP connections for email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 May 2025
Updated 16 Aug 2025
8 min read
One of the more persistent myths in email deliverability is the idea that your sending IP address, the server that initiates the connection to send email, must also be configured to accept incoming SMTP connections. I hear this question quite often, and it stems from a misunderstanding of how email flow works and what various recipient networks look for.
Let me clarify this straight away: a sending IP address typically does not need to accept incoming SMTP connections for good email deliverability. The functions of sending and receiving email are distinct, and best practices actually advocate for separating these roles for security and operational efficiency. While some smaller, less sophisticated recipient servers might perform this kind of check, it is not a widespread or necessary requirement among major email providers.
The focus should instead be on the domain itself and its associated DNS records, which play a far more critical role in how your emails are perceived and ultimately delivered.
The sending IP's primary role in deliverability
When your email server sends a message, it acts as a client initiating an outbound connection to the recipient's mail server. This sending IP needs a good reputation to ensure your emails aren't flagged as spam or rejected outright. Its primary job is to establish that connection, authenticate the sender where required, and transfer the email data.
A crucial aspect for any sending IP (or IP address) is having a valid reverse DNS (PTR) record. This record translates an IP address back to a domain name. Many Mail Transfer Agents (MTAs) perform a PTR lookup on the connecting IP address to verify its legitimacy. Without a proper PTR record, your emails are much more likely to be rejected or sent to the spam folder, even by large providers like Google. It is considered a fundamental best practice for email sending.
The reputation of your sending IP is paramount. This reputation is built over time based on your sending practices: low bounce rates, minimal spam complaints, and consistent volume. If you're a high-volume sender, a dedicated IP address can offer more control over this reputation, as it's not shared with other senders whose practices might negatively impact you.
However, none of these requirements necessitate that the sending IP itself accept inbound mail connections.
The myth of the inbound sending IP requirement
While the sending IP focuses on outbound traffic, the ability to receive mail is handled at the domain level, specifically through MX (Mail Exchanger) records. These DNS records specify which servers are responsible for accepting incoming mail for a particular domain. When a recipient's server receives an email, it looks up the MX records for the sender's domain (the From domain, or sometimes the MAIL FROM domain) to ensure it can actually receive replies or bounces. This is a standard check and a legitimate expectation for any domain used for sending email.
Some might confuse this MX record check with an inbound connection attempt to the sending IP. They are distinct. A major reason for this confusion could be that in smaller setups, a single server might handle both sending and receiving. However, in larger, more robust email infrastructures, these functions are almost always separated.
The idea of a recipient server connecting back to your sending IP on port 25 before accepting your email is generally considered an outdated or ineffective check. Most major Internet Service Providers (ISPs) and email providers understand that modern email architectures compartmentalize sending and receiving roles. Requiring the sending IP to accept incoming connections would be a pointless exercise for them and could even indicate a misguided approach to email security or validation on their part.
This also applies to situations where your origination IP (where the email client connects) and your outbound IP (where the email leaves your network) might differ. The critical factor for deliverability is the outbound IP's reputation and its associated DNS records, not its ability to receive inbound connections. You can find more details on this topic in our article, Should my origination IP and outbound IP be the same?.
Architectural separation and security
Sending IP address
Purpose: Initiates outbound SMTP connections to deliver emails to recipient servers.
Incoming connections: Not required to accept incoming SMTP connections. Often firewalled for security reasons.
Separating the roles of sending and receiving email servers is a fundamental security and architectural best practice. A server dedicated solely to sending outbound mail can be optimized for that purpose, with its firewall configured to block all inbound connections on port 25, except for necessary administrative access. This significantly reduces the attack surface and helps prevent the server from being exploited for spam relaying or other malicious activities.
If a server's sending IP address were required to also accept incoming connections, it would complicate firewall configurations and introduce unnecessary security risks. Email Service Providers (ESPs) and large organizations commonly employ this architectural separation, managing distinct sets of IPs for outbound marketing, transactional emails, and inbound mail reception.
When you encounter claims or observations that suggest a sending IP needs to accept inbound connections, it is almost certainly a misinterpretation. The real check is at the domain level, verifying that the domain (not the specific sending IP) can receive mail through its MX records.
This distinction is vital for maintaining a secure and high-performing email infrastructure. If a recipient system tries to connect back to your sending IP, and that connection fails because the port is closed (as it should be for security), it typically will not impact your email deliverability. The system will then proceed to check other, more relevant factors.
What truly matters for email deliverability
Instead of checking whether your sending IP accepts inbound connections, recipient mail servers primarily focus on the authenticity and legitimacy of your domain and its associated email authentication records. These are the true indicators of a trustworthy sender.
The key checks that recipient servers perform include:
SPF (Sender Policy Framework): Verifies that the sending IP is authorized to send email on behalf of your domain. If the SPF record is not correctly configured, the email may be marked as spam or rejected.
DKIM (DomainKeys Identified Mail): Adds a digital signature to your emails, allowing the recipient server to verify that the email has not been tampered with in transit and truly originated from your domain.
DMARC (Domain-based Message Authentication, Reporting, and Conformance): Builds upon SPF and DKIM, providing instructions to recipient servers on how to handle emails that fail authentication, and allowing you to receive reports on email authentication performance.
MX records: As mentioned, recipient servers often check that the sending domain has valid MX records, confirming its ability to receive mail. This is important for replies, bounce management, and overall domain credibility.
Proper configuration of these records is crucial for email deliverability in 2024 and beyond, especially with new requirements from major mailbox providers like Microsoft Outlook and Google Workspace. Focusing on these foundational elements will yield far greater improvements in your inbox placement than worrying about an inbound connection to your sending IP.
Final thoughts
Ultimately, the primary concern for any email sender should be establishing and maintaining a strong sender reputation and ensuring all necessary DNS records and email authentication protocols are correctly implemented. Your sending IP's ability to accept incoming connections is not a factor in this equation for modern, sophisticated email systems.
Focus your efforts on proper sender authentication, list hygiene, and content quality. These are the cornerstones of successful email deliverability and will have a tangible impact on your inbox placement rates.
Views from the trenches
Best practices
Ensure your sending domain has a valid MX record, allowing it to receive bounces and replies.
Always set up a correct PTR (reverse DNS) record for your sending IP address, as this is a fundamental requirement.
Implement SPF, DKIM, and DMARC to authenticate your emails and enhance sender trust with recipient servers.
Common pitfalls
Mistaking the need for a domain's MX record to accept mail with a requirement for the sending IP to accept incoming connections.
Configuring firewalls on sending IPs to allow inbound SMTP, which is unnecessary and creates security vulnerabilities.
Overlooking the importance of PTR records while focusing on irrelevant checks.
Expert tips
Sender authentication (SPF, DKIM, DMARC) and positive sender reputation are the most critical factors for deliverability.
Modern email infrastructure separates sending and receiving functions for security and efficiency.
If a recipient server attempts an inbound connection to your sending IP, it is likely an outdated or irrelevant check.
Expert view
Expert from Email Geeks says the primary check is whether the domain has an MX record and accepts mail, not whether the sending IP accepts incoming connections.
2023-02-15 - Email Geeks
Expert view
Expert from Email Geeks says that checking if a domain can receive mail is done at the domain level, not the IP. It is bad practice to require a sending IP to receive traffic for security reasons, but some might check if the domain can receive mail by trying to connect to its MX record.
