Hello.
Whilst this is partially outside of our support level - we do know how Exchange works so I will try and answer most of your questions.
1) Not exactly. If you are still colleting from us via Exchange, this will be POP. But if you are referring to mail sent directly to your mail server, this is neither POP3 or IMAP. It is generally referred to as a SMTP feed.
2) General rule of thumb is that you must have some sort of rDNS - spammers generally use servers that may not have rDNS hence why alot of places (including us) reject IPs that do not. The computer company should have known and advised you of this prior - as you say.
3) Yes. However, if you go into Exchange manager within windows - you may be able to send them again. It all depends if the remote server has rejected them with a failure error code or a complete reject code. You should have received them back if they have been rejected completely.
4) No. Your mailserver is pointed to office.domain.com (which we would recommend setting your reverse DNS to - although this is not that important). The website will be pointed to domain.com or
www.domain.com which do not match the mailserver record.
5) I would change the reverse DNS that is currently set to 'domain.com' to 'office.domain.com'. Possibly look at moving the domain to us completely to reduce the amount of people you have to deal with.
In regard to how roubst the setup is - I think we advised you on this the other day about having a mail server on a DSL line.... We can provide a secondary MX service to you so that if the master mail server is down - it will attempt to re-deliver over the course of 7 days. Contact sales for a quote on this.
Matt