can't send or receive emails

Att.net emails worked for 15+ years. Stopped suddenly. Couldn't login but got error message "NOT FOUND ON ACCELERATOR". If I tried logging in from att.yahoo.com yahoo returns my address is unknown.

Spoke to 3  different departments, reset my password 3 times and finally able to login and view my profile. If i attempt to view emails I get the same error message "NOT FOUND ON ACCELERATOR". The last person I spoke to said he went as far as he could and suggested I try 

ATT.COM/emailsupport which turned out useless. All I get is circular reasoning and no answer.