Summary
The common assumption that a sending IP address must also accept incoming SMTP connections for optimal email deliverability is largely a misconception. While some older or smaller recipient networks might perform checks that could appear to validate inbound SMTP on a sending IP, the prevailing consensus among experts and major internet service providers (ISPs) is that such a requirement is unnecessary and, in many cases, impractical. The crucial factor for deliverability is that the sending domain (specifically the Mail-From or envelope-sender domain, RFC5321.From) has valid MX records and is capable of receiving mail, rather than the specific IP address from which the mail originated.
Key findings
- Domain-level checks: Email deliverability checks primarily focus on the sending domain's ability to receive mail, not the sending IP address.
- MX record importance: Recipient mail servers (receiving mail servers) will check for valid MX records for the sender's domain (RFC5321.From) to confirm it can receive mail. This is a standard and expected practice.
- IP-based checks are rare: Requiring the sending IP itself to accept incoming SMTP connections on port 25 is not a common practice among reputable ISPs or major email providers. Such checks are considered largely useless and misguided.
- Industry practice: Many large email service providers (ESPs) and companies operate with separate infrastructure for sending and receiving emails, meaning their sending IPs do not accept inbound connections.
Key considerations
- Focus on domain deliverability: Ensure your sending domain has proper MX records and is configured to receive mail. This is crucial for maintaining good sender reputation and compliance.
- Security best practices: Separating sending and receiving services (potentially on different IPs or machines) can be a good security practice. Having a sending IP that doesn't accept inbound mail isn't inherently problematic for deliverability.
- Limited impact: While very few niche recipient servers might perform an IP-based inbound SMTP check, their impact on overall deliverability is minimal. Focus on standard authentication (SPF, DKIM, DMARC) and sender reputation.
- Check MX records: Regularly verify that your domain's MX records are correctly configured and pointing to active mail servers. This is far more important than the sending IP's inbound capability. You can learn more about this in our guide on whether outbound email server IPs need port 25 connectivity. Additionally, explore if your dedicated sending domain should receive emails. For a deeper dive into incoming and outgoing server configurations, refer to MailServerGuru's detailed guide on incoming and outgoing mail servers.
What email marketers say
Email marketers often encounter various technical considerations related to email infrastructure and deliverability. While some might initially wonder about the necessity of a sending IP accepting incoming SMTP connections, the collective experience points towards this being a non-standard or low-priority requirement. The focus for marketers generally remains on maintaining a strong domain reputation, proper authentication, and engaging content to ensure messages reach the inbox. They observe that major ESPs do not enforce such a strict IP-based inbound rule.
Key opinions
- Common misconception: Many marketers initially question this, but quickly find that the requirement for a sending IP to accept incoming SMTP is not widely supported or practiced.
- Focus on domain: The primary concern is the domain's ability to receive mail via its MX records, not the sending IP.
- Limited impact of strict receivers: Even if a few small recipient domains perform an IP-based check, it does not significantly affect overall campaign deliverability, as large providers don't use this metric.
- Real-world observations: Tests confirm that emails are accepted by domains even when sent from IPs that explicitly deny inbound SMTP connections.
Key considerations
- Prioritize core deliverability: Marketers should focus their efforts on sender reputation, content quality, list hygiene, and robust authentication protocols (SPF, DKIM, DMARC).
- Understand the difference: Distinguish between domain-level requirements (MX records) and IP-level requirements, recognizing that the latter is largely irrelevant for sending email.
- Optimize setup: Ensure your email sending infrastructure is optimized for outbound mail, potentially using a shared IP address or a dedicated one, without necessarily configuring it for inbound receipt. This is a key aspect of fixing common spam issues. For more general guidance on email sending, consult resources like HighLevel's email sending guide and best practices.
Marketer view
Email marketer from Email Geeks questions whether a sending IP address also needs to accept incoming SMTP connections for best practice setup or email deliverability. They wonder if recipient networks might judge a sender based on the IP they receive mail from not allowing inbound SMTP.
Marketer view
Marketer from Email Geeks affirms their stance that there is no IP check for inbound connections on the sending IP and notes that others in the discussion share this opinion, which they find reassuring.
What the experts say
Experts in email deliverability and infrastructure are very clear on this topic: the ability of a sending IP address to accept incoming SMTP connections is not a standard or necessary requirement for good email deliverability. They emphasize that recipient checks are performed at the domain level, specifically verifying that the sender's domain (RFC5321.From) has valid MX records and can receive mail. Requiring a sending IP to also be an inbound mail server is considered an outdated or misguided practice that would introduce unnecessary security and architectural complexities.
Key opinions
- Domain, not IP: The critical check for receivers is whether the sending domain (via its MX records) can receive mail, not if the specific sending IP accepts incoming SMTP.
- Useless check: Experts widely agree that requiring the sending IP to accept port 25 connections is a meaningless check that only very few, misguided domains might perform.
- Bad practice to require: It is generally considered poor practice to require sending IPs to receive mail due to security and architectural reasons (e.g., separating services on different machines).
- Industry norm: It's a long-standing practice for legitimate senders to use IPs dedicated solely to outbound mail, which do not accept inbound connections.
Key considerations
- Prioritize domain health: Ensure your sending domain has proper DNS records, including MX records, and that they are resolvable and functional.
- Architectural separation: Embrace the industry standard of having separate IPs or servers for sending and receiving mail for better security and management.
- Educate about misconceptions: Help dispel the myth that inbound SMTP on a sending IP is necessary for deliverability, reinforcing the focus on domain-level checks. Understanding the impact of PTR records and HELOs is more relevant. Additionally, consider whether an A record is needed for email sending domains. For a broader perspective, EmailLabs discusses how email sender reputation affects deliverability, which is a far more impactful factor.
Expert view
Expert from Email Geeks clarifies that the relevant check is whether the domain has an MX record and if that MX record accepts mail; the sending IP itself is not the subject of this check.
Expert view
Expert from Email Geeks explains that checking if something can receive mail is a domain-level function, not an IP-level one. They add that requiring the sending IP to receive mail would be bad practice for security reasons, favoring dedicated services on different machines.
What the documentation says
Technical documentation and RFCs (Requests for Comments) define the protocols governing email communication. These documents clearly distinguish between the roles of mail transfer agents (MTAs) for sending and receiving, and the significance of DNS records like MX records. While an IP address must be capable of establishing an outbound SMTP connection to send mail, there is no mandate within core email protocols that the same IP must also accept inbound SMTP connections for email deliverability or validation purposes. The focus for inbound mail routing is explicitly on the domain's MX records.
Key findings
- RFC standards: RFCs define SMTP for outbound mail and specify MX records for inbound mail routing; they do not link the sending IP's inbound capability to deliverability.
- Separate functions: The architecture of email often involves distinct servers or IPs for sending and receiving, a design choice fully supported by standards.
- MX record lookup: Inbound mail delivery relies on a DNS lookup for MX records associated with the recipient domain, not a reverse lookup on the sender's IP for inbound SMTP.
- No deliverability impact: Lack of inbound SMTP on a sending IP has no documented negative impact on email deliverability according to major protocol specifications.
Key considerations
- Understand SMTP basics: Familiarize yourself with SMTP protocols, which primarily govern outbound mail transfer. Mailgun's explanation of SMTP relay provides valuable insight into this.
- DNS configuration: Ensure your domain's DNS is correctly set up with MX records for inbound mail, even if your sending IPs are separate. This is covered in our simple guide to DMARC, SPF, and DKIM.
- Adherence to RFCs: While some interpretations vary, core RFCs do not require sending IPs to accept incoming connections. See our blog on what RFC 5322 says versus what actually works.
Technical article
Documentation from Mailgun's blog on SMTP commands and best practices defines SMTP as the protocol for sending emails. It outlines the sequence of commands used for outbound mail transfer, without mentioning any requirement for the sending IP to accept inbound connections.
Technical article
SiteGround's tutorial on email protocols (POP3, SMTP, IMAP) details the distinct functions of incoming and outgoing mail servers. It specifies that SMTP is for sending, while POP3 and IMAP are for receiving, implying a separation of roles and no requirement for an SMTP sending IP to be an IMAP/POP3 receiving server.
Related resources
9 resources
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What is a shared IP address and how does it affect email deliverability?
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