smtp.web.de fails to send emails

Recently a friend of mine, who uses my mail service, complained about e-mail not being received from users of WEB.DE. The sender received an error mail with following content:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address
failed:

  <nix@example.com>

domain name system error:
domain has no mail exchangers

Domain Name System error? But everything is configured just fine and my mail service works, even after double checking!
After a lot more testing I determined that this error is reproducible, but only with a special setup, so most users won't encounter this issue. This special case is as follows:

The sender uses a mail client - not the webmail interface - which connects to the server smtp.web.de and tries to send a mail. The recipient's email domain (example.com) has following DNS entries:

• example.com.          MX 10 mail.example.com.
• mail.example.com.     CNAME server1.example.com.
• server1.example.com.  A 192.168.0.1
  (this is just an example, not the real address)

The not working part in this setup is the CNAME entry. For some reason the server smtp.web.de does not get the real mail server (address) and refuses to send the email with the error described above. Removing the CNAME entry and putting server1.example.com directly to the MX entry solved this issue.

But this is just a workaround and does not solve the real problem: why can't the server smtp.web.de retrieve the address if it's "sym-linked" with CNAME?

[Update] RFC 2181 section 10.3 states:

The domain name used as the value of a NS resource record, or part of the value of a MX resource record must not be an alias. Not only is the specification clear on this point, but using an alias in either of these positions neither works as well as might be hoped, nor well fulfills the ambition that may have led to this approach. This domain name must have as its value one or more address records. Currently those will be A records, however in the future other record types giving addressing information may be acceptable. It can also have other RRs, but never a CNAME RR.

Oops. My bad. Note to myself: RTFM.