Lack of help from eM Client

My entire email system has stopped working for no apparent reason - just getting .'Server says
“(AUTH) Authentication failed.”
eM Client has said the problem is with my server, however, having spoken to them, they have advised that there is no fault at there end and all my other systems are working OK.
I have had little to no help from eM Client and would like to know how to transfer all my email addresses to another supplier.
Any recommendations would be most helpful.

My entire email system has stopped working for no apparent reason - just getting .'Server says
“(AUTH) Authentication failed.”

What mail server is this ?

Is it a POP, IMAP, Exchange, Office 365 or iCloud account?

Does your server have a passwordless OAuth token, Or use a legacy username and password ?

Also what is your Incoming Server Address, Server Port & Security Policy.

Lastly what version of eM Client do you have ? and do you have Windows or Mac & OS ver ?