Workmail error whille logging IN

Hi, you should check your Workmail Access Control Rules: they are very often the root cause of such 403 (Forbidden) errors. You may have one that blocks users

See https://docs.aws.amazon.com/workmail/latest/adminguide/access-rules.html

Administrators can apply access control rules that allow or deny access 
based on the following criteria:

Protocols – The protocol used to access the mailbox. Examples 
include Autodiscover, EWS, IMAP, SMTP, ActiveSync, Outlook for 
Windows, and Webmail.

IP addresses – The IPv4 CIDR ranges used to access the mailbox.

Amazon WorkMail users – The users in your organization that are 
used to access the mailbox.

Impersonation roles – The impersonation roles in your organization 
that are used to access the mailbox. For more information, 
see Managing impersonation roles.

Hope it helps! Didier