[y@w]

Originally Posted by phoenix

The issue is with using a CNAME and incorrect set-up of DNS records.

Beware Using CNAME and MX at the Same Time
Why You Shouldn’t Mix CNAME and MX
How Best to Use a CNAME

While using a CNAME can work and it's use is rather ambiguous in the RFC, it's far better to avaoind it's use when pointing to a mail server.

Agreed, but we're not talking about using the CNAME as the MX record. In this case they are trying to use Zdesktop to connect to Yahoo mail. Zdesktop needs to get to the mailbox server, not the MTA which appear to be different hosts (I would assume they are). Yahoo does not use a CNAME in its MX records, however the IMAP and SMTP servers for public use do appear to be CNAMEs.

The one the OP mentioned was zimbra.imap.mail.yahoo.com which the mail client actually has to follow a trail of CNAMEs to get to an A record for. If the problem lies in using a CNAME to get to a mailbox server, I would have thought this would be a lot bigger issue for the product. Both Yahoo Mail and Gmail use CNAMEs for their IMAP servers. But again, they don't use CNAME records for their MX records. It very well could be that the issue in this case was something machine-specific for this person that caused it to not be able to, but retrieving mail from a host using a CNAME DNS entry is quite common.