There are tons of reasons an email could be trapped within the spam filter, but tons of them are fairly technical and rare.
Sending from a Blacklisted Mail Server (Host)
If your mail server has been blacklisted, this is often getting all of your emails blocked by spam filters. you ought to periodically check if your mail server has been blacklisted, and also found out alerts.
If you've got images in your email signature that are hosted on a website that has been blacklisted, then your email signature is probably going to cause your whole email to be marked as spam.
Attachments are usually scanned for viruses, so ensure your files are clean.
Font Colours and Sizes
If you've got fonts which are different sizes and colors, which will likely trigger spam filters.
Too Many Special Characters
Try to not use too many exclamation marks, question marks and the other special characters like hashes, percentage symbols.
Contact ATT Support technical team and learn about issues and its solutions. ATT Email technical team will give instant solutions by email team. ATT Email Support Resolve every type ATT Email Problems and you will learn about issues and resolve on your own. Dial ATT Mail Support toll-free number 1-800-982-1502 and get the best email support services 24/7 and click on the website link given.
Reference Url:- https://thomasdean8285.tumblr.com/post/190164379020/why-emails-are-flagged-as-spam