receiving mail

[Pieter Donche]

Newly installed FreeBSD-7.0.
Sending mail to other hosts works. 
Receiving mail does not:

Between host1 (non-freebsd) and host2 (freebsd 7), sending a mail from 
host1 to host2 (both in domain  .domain.topdom) results in message 
'Returned mail: see transcript for details":

    ----- The following addresses had permanent fatal errors -----
<user at host2.domain.topdom>

    ----- Transcript of session follows -----
550 5.1.2 <user at host2.domain.topdom>... Host unknown (Name server:
mailhost.domain.topdom: host not found)
-------------

host1 and host2 are both behind a firewall, but they are in the same
zone (so firewall rules do not apply), both are even connected to same
switch and their IPs are in the same subnet.
Both have fixed IP address (not DHCP)

mailhost.domain.topdom does not exist in DNS, but host1 can send
mail to other mailservers all over the world.

host2(FreeBSD):

/etc/nsswitch.conf
hosts:  files dns

/etc/resolv.conf
domain  domain.topdom
nameserver UUU.VVV.WWW.ZZZ   (correct ip address of a DNS nameserver)

DNS is fully working on host2 (and on host1)

According to the FreeBSD handbook I should be able to receive mail ?
What can be missing here, how to remedy?