we use office365 Exchange Online for our helpdesk email account.
WHMCS helpdesk retrieves these mails via pop3 authenication.
Microsoft has announced the retirement of Basic Authentication for EWS, EAS, IMAP, POP and RPS to access Exchange Online.
Beginning October 13, 2020, we will retire Basic Authentication for EWS, EAS, IMAP, POP and RPS to access Exchange Online. Note: this change does not impact SMTP AUTH. There are several actions that you and/or your users can take to avoid service disruptions on client applications, and we describe them below. If no action is taken, client applications using Basic Authentication for EWS will be retired on October 13, 2020. Any application using OAuth 2.0 to connect to these protocols, will continue to work without change or interruption.
Please implement OAuth 2.0 in WHMCS to access these mailboxes after October 13, 2020