To expand upon this for the benfit of others in search for an answer on whether a DNS wildcard entry takes precedence over a "normal" subdomain entry, it looks like it varies depending on implementation:
There is a cPanel forum post if you Google "wildcard DNS precedence" which shows that a wildcard entry will take precedence, which isn't compliant to RFC 4592: http://tools.ietf.org/html/rfc4592
Other sites suggest proper behavior for their services
I haven't tested this in practice, but my understanding is that a wildcard is supposed to match requests for non-existent names only, so in your example, the mail subdomain should be unaffected by the wildcard entry.
There's this RFC which explains it in full: rfc4592
The cPanel Technical Sales adviser may simply be incorrect, or cPanel implements wildcard DNS in a faulty manner.