NMC2 (AP9631) SMTP fails to send via Ionos, workaround uses gmail

[BillP]

On investigation, this error appears to have been happening since early April 2020 (firmware 6.8.2) and occurs with firmware 6.8.8, 6.9.4 and 6.9.6.

Mail is configured to use the "custom" server, by recipient (authenticated SMTP, port 465, implicit SSL/TLS). Only one recipient is defined.

Attempts to send mail via smtp.ionos.co.uk (Ionos, formerly 1 and 1) fail with the following error (description from the Ionos web pages):

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554 Transaction failed, reject due to policy restrictions

Problem:

The email was rejected by the IONOS SMTP server because the standards defined in RFC 5321 and RFC 5322 were not adhered to.

Solution:

The email client must meet the following criteria:

• The header lines of the email must contain exactly one valid "Date" line according to RFC 2822.
• The following header lines may only exist once: From, Sender, To, CC, Subject
• The header lines mentioned must be syntactically correct

Note: The To and CC lines can contain multiple recipients.

In accordance with the recommendations from RFC7103, incorrect messages are not automatically corrected by IONOS, otherwise DKIM signatures become invalid. For more information about DKIM signatures, see the IONOS article Configuring DMARC Records for a Domain.

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As a workaround, configuring the mail server to use smtp.gmail.com with authenticated SMTP, implicit SSL/TLS and port 465 works.

I recall something similar a few years ago, where the NMC2 mail server did not send via what was then 1and1 using authenticated SMTP with SSL/TLS. That was resolved by a firmware update. It would seem that the current firmware (since 6.8.2) has a problem in the way that SMTP messages are sent, such that the Ionos SMTP server rejects them.