Hello and thank you for you reply. Please find the Dashboard screenshot and also the mail settings. I've used one account as an example. However all 3 accounts behave the same, 2 x Gmail and 1 x Office 365. I can receive mail using IMAP from all accounts using oAUTH2. Same for Outgoing mail server, I used oAuth2 and tests successfully, but no delivery of the mail occurs from any account to any destination.
I have reauthenticated several times over for both IMAP and SMTP with success messages. No mail errors in log and still no delivery.
The most recent log is 18 Jan - PHP Fatal error: Uncaught Error: Class "Mailer" not found in. But I have been testing vigorously for days, so if were related errors I guess I'd be seeing heaps error here for today, but nothing since 18th.