Two comments about the output you've posted. You have two NS records, they should use an IP address not an FQDN. The hosts file is incorrect, the following line needs to be changed from:
to the following:
192.168.100.5 mail.domain.com mail domain.com
Make those changes and see if anything changes.
192.168.100.5 mail.domain.com mail
One comment about the ports you have open in the router. You should not be using ports 80 & 110 on a server that's accessible to your users (both are a security risk), you should be using 443 https (instead of 80) and pops 995 (instead of 110) - in fact I'd suggest they use IMAP instead of pop for a better user experience (they also won't remove mail from the server) or even Zimbra Desktop.
Apart from the messages with attachments is there no other activity on the server at that time? Are these a total of 106MB of attachments or one attachment in one email?
I forgot to ask if this problem has just started or have you had it for a while? If it's just started, has anything been changed or updated on your server?