MX records should be names, not addresses. The address will be looked up from the name using A or AAAA records.
So you should have something more like:
mail.localhost.localdomain. IN A 192.168.1.15 mail.localhost.localdomain. IN MX 10 mail.localhost.localdomain.
Note that only A and AAAA records are used: the FQDN referred to by an MX entry should have valid A and/or AAAA records, CNAME records will not be used.
Also, the message suggests it is looking for an MX entry for localhost.localdomain not mail.localhost.localdomain - so you may need:
localhost.localdomain. IN MX 10 mail.localhost.localdomain.
instead or as well. If you are wanting to serve mail for <users>@mail.localhost.localdomain
and not <users>@localhost.localdomain
then you may have your hostname set wrong elsewhere (the Zimbra admin/install forums or serverfault.com may be a better place to ask about that (in fact this question would better sit on serverfault.com than superuser.com IMO)).