Website is Blacklisted as Malware

My business website is experiencing what seems to be an alarmingly common issue wherein anybody using AT&T is denied access.  The prompt claims malware (see image).  I'm praying that a streamlined solution has been established, given how many similar unresolved threads I have come across here in the forum.  Best I could deduce was that the postmaster needs contacting, though the links provided to do so carry an alert of their own, warning they lead somewhere that is not affiliated with ATT&T.  Feels awful sketchy.  Can anyone provide safe, necessary steps in order to whitelist my site?  This is inflicting a significant blow to our business operations.  Thank you.