The test recipients are using Outlook? - YES
Are you using the internet security suite? - What do you mean?
Are you using any antispy or antimalware apps? No
Which firewall do you use? - Sonicwall TZ-210. It has been in place for over 1 year. No changes.
What port are you using? POP:110 SMTP: 25
Using SSL? NO
Does your host provide multiple ports? No Idea, but unlikely
Switching ports and using SSL can get around sneaky scanners (that error is almost always due to scanners - its actually the only cause i am aware of).
Did you also test with messages without attachments? YES - Messages go through without attachments
Try sending a no-attachment message then one with an attachment a little bit later -
do you get the error before or after the no-attachment message is downloaded? Both test recipients receive the error before the message is downloaded.
(I'm not sure it will tell us anything though.)