There can be multiple reasons for this however it can be anything from the below pointers.
Warmup has been stopped due to InvlidAuthenticationToken problem, Which means either token or credentials are expired or changed from user end. When there is problem with authentication, we receive a common error code. Which is "InvalidAuthenticationToken".
SMTP and IMAP Returns the same. There can be multiple email accounts are disabled for the same account, so there has to be some update on users SMTP/IMAP Server.