Why are there two domains in the 'Mail From' field of an email header, and how does it affect deliverability?

Email marketer from EmailOnAcid describes that the Authentication-Results header provides a summary of the authentication checks performed on the email. It explains different mechanisms like SPF, DKIM and DMARC that were applied. The header can display several domains that participated in the process of email verification. If the SPF and DKIM alignment don't line up then it can cause problems.

Email marketer from GlockApps shares that domain reputation significantly impacts deliverability. If one of the domains involved (e.g., the sending domain or a domain used in redirects) has a poor reputation, it can negatively affect whether the email reaches the inbox. Monitoring domain reputation is crucial for maintaining good deliverability.

Email marketer from StackExchange discusses that the 'MAIL FROM' domain is crucial for bounce handling. If this domain isn't properly configured to receive bounces (e.g., with a valid return-path), it can lead to deliverability issues as ISPs may see the sender as not handling bounces correctly.

Email marketer from Reddit explains that multiple domains can appear due to SPF, DKIM, and DMARC authentication processes. Each domain serves a different purpose in verifying the sender's identity, but if one domain fails authentication, it can impact deliverability.

Email marketer from Mailhardener explains that the presence of multiple domains in the 'Authentication-Results' header reflects the different domains involved in the email's authentication process. The smtp.mailfrom domain is the envelope sender verified by SPF. Additional domains could be related to DKIM signatures. These domains collectively contribute to the receiving server's assessment of email legitimacy.

Email marketer from EasyDMARC notes that if the HELO domain does not match the 'MAIL FROM' domain and SPF fails, it can hurt deliverability. A mismatch raises red flags for receiving servers, suggesting potential spoofing or other malicious activity. Alignment between these domains is ideal.

Expert from Email Geeks explains that the second domain in the authentication results might be the receiving domain, and it's unlikely to be included in the reputation calculation.

Expert from Email Geeks explains that smtp.mailfrom is the envelope from, which is verified by SPF. He also suggests the second domain might be the HELO, which SPF also checks against. Mentions that ISPs consider other domains in the transaction when assessing reputation, and you can check the 'received' header to see the HELO.

Expert from Spam Resource discusses how Sender ID, an older email authentication method, uses the Purported Responsible Address (PRA) to determine the sender's identity. It explains that this is often based on the domain in the 'From:' header, however, its impact on deliverability is not as significant as SPF, DKIM or DMARC.

Expert from Word to the Wise explains that the HELO/EHLO domain is checked during SPF authentication. While not directly in the 'Mail From' field, the domain presented in HELO plays a role in authentication and can impact deliverability if it doesn't align with the sending domain's SPF record.

Expert from Email Geeks confirms that the second domain is the authserv-id, which is meaningless and not going to be the spammy domain in a real hotmail header, it will be hotmail.com

Documentation from RFC Editor explains that SPF uses the domain in the 'MAIL FROM' (envelope sender) for authentication. The receiving server checks if the sending server is authorized to send email for that domain. It doesn't directly address two domains but establishes the importance of the 'MAIL FROM' domain for authentication.

Documentation from RFC Editor explains that the 'MAIL FROM' command in SMTP specifies the reverse-path, which can be a null path (<>) or an email address. While it doesn't directly address two domains, it establishes the syntax for the envelope sender.

Documentation from Microsoft Learn explains that the Authentication-Results header can contain multiple authentication checks, potentially showing different domains involved in the email's path. One domain is the 'smtp.mailfrom' which corresponds to the MAIL FROM and is verified by SPF, while another domain may relate to DKIM signature verification, and a final one being the receiving domain or authserv-id. Microsoft’s systems use authentication checks to assess the validity of an email sender. Seeing multiple domains reflects the various stages and services involved.