Special Providers¶
The SMTP protocol is clearly defined in RFC standards:
Most email providers and infrastructure conform to these standards. Unfortunately, however, there are some providers that do not adhere to standards.
Consequences of lack of compliance to standards can result in:
- Deliver-ability issues
- Issues with SMTP email verification processes
In this document, we identify known providers that create issues due to lack of compliance to RFC standards.
Office 365¶
Standards Violations:
- RFC821 section 3.1. MAIL If the recipient is unknown the receiver-SMTP returns a 550 Failure reply.
Since mid 2016, Email Hippo customers have reported some Office 365 hosted domains as reporting 250 OK during email verification processing. Subsequently, on customers attempting to send emails to these email addresses, Office 365 has responded with hard bounce NDR.
For our customers that rely on integrity of our email verification processing to determine deliver-ability quality, receiving hard bounces on mail send is unacceptable.
Email Hippo therefore, in the interests of reporting accurately, has taken the decision to label all Office 365 hosted domains as Unverifiable / UpredictableSystem.
Yahoo!¶
Standards Violations:
- RFC821 section 3.1. MAIL If the recipient is unknown the receiver-SMTP returns a 550 Failure reply.
Yahoo mail services attempt to manage incoming spam with a blended approach of:
- Repuation based assesment
- Non-standard imlementations of the SMTP protocol
- Velocity and traffic management
- Proprietary white-listing
Sending email to Yahoo! or performing email verification processes can be problematic based on the non-standard Yahoo! implementations of the SMTP protocol and ancillary services.
Email Hippo delivery partial coverage of Yahoo! based email addresses. However, due to reasons stated above, we are unable to state or underwrite any specific coverage statistics for this domain.