Beyond me by now, but will write about. After 3 phone calls, and very long wait time I still have the same problem.
Only with OUTGOING mail. I have COX. NET account, do not use OUTLOOK. As of 2 days ago most OUTGOING mail just fly away, and NO attachment works at all. Small only about 900 KB.... NOTHING goes out. Adjusted Microsoft Edge Advanced settings, took off ADOBE FLash, put back ADOBE flash, adjusted cookies.... no result. Was sent a link to use IPHONE. IPHONE works, do not have to SYNC it. WIN10 what I have, and no Outlook. Support people just ask one more question instead of reading my answers. CLASSICAL email, only COX, no OUTLOOK, adjusted setting back and forth on MICROSOFT EDGE advanced settings. Then they just take a note. Great.