E-mail from Exchange server being flagged as spam
I've recently set up an old Dell Optiplex running Windows Server 2003 and Exchange 2003 at home for home file server duties, as well as providing a place to host web/e-mail service for my domain.
All was well for about a week or so, then over the last few days, e-mail sent to others seems to get rejected or flagged as spam by recipient mail servers. A friend noticed that when I sent him an e-mail to his Hotmail account, my message got flagged as junk. I have just tested with my Google Mail account and mail that gets sent to there is also flagged as spam.
I have checked to see if my domain or IP address is blacklisted using MX Lookup Tool - Check your DNS MX Records online - MxToolbox and all looks well.
Does anybody have any ideas as to what the hell could be causing this and how to fix it?
Any help is much appreciated!