[exim] CNAME and recipient rewriting

Hi,

I recently got notified from a mailing list which is mangaged with
ezmlm that E-Mails could not be delivered because my mail server
rejected them. It turned out that the SMTP server or ezmlm itself
rewrote the recipient of the envelope from bob@??? to
bob@???, because example.com was a CNAME for b.example.com.

Is such rewriting compliant with any standard (according to the
rejectlog no other E-Mails got rejected for this reason)?

Regards,
Matthias-Christian