Do you use anti-spam software? A client used anti-spam software that had its own SMTP engine. When it bounced/rejected spam it was seen as outgoing traffic which got them blacklisted repeatedly. AV software will not cause this. All you need do, if this is the case, is set the anti-spam NOT TO reject spam and/or send a letter to the sender. Instead route the spam to a SPAM folder and set it to delete every few days.
Also, Malwarebytes will help you get a handle on any spyware that may have a toehold on your system.
There are plenty of good anti-spam applications out on the web that can be had for little to nothing.