I have been through blacklistings before.
It can be more than the general blacklist.

But get down to individual servers on private networks such as corp or government email
where they subscribe to blacklist services outside of just spam but place you in what they want to block , such as sports or entertainment sites that they elect.
Good luck on correcting that.

UBB.threads 7.7.0 has been released and is now available to download in the Member Area.

Primary Changes In This Release

• UBB.threads works with PHP 5.4 - 7.3.
• New MySQLi database connection library.
• WYSIWYG editor for composing HTML formatted email in the Mass Mailer.
• Advanced mail configuration settings for setting MTA, encryption type, and port.
• Member Editor now provides an inline GEO-IP lookup.
• Style Editor now allows styling of the Member Avatars.
• News Portal has been updated with a modern entry page layout.
• Members with permission can create topics for the News Portal from any forum.
• Several basic News Images are now included with UBB.threads
• Followed/Unfollowed threads and forums are now better represented.
• All PHP script files have been reformatted to be consistent with a basic coding standard.
• Cleanup of many language strings, templates, and CSS files.
• Any code and page validation errors that were found, were fixed.
• Updated libraries and assets.

Version Discussion and Changelog