OK, have sanity checked this with another colleague and has got me stumped.. hoping someone else has experienced this.

Outlook 2016, configured (using auto discovery) 1x outlook.com account, obviously this uses EAS to send/receive/sync etc.

Compose new mail, click send, goes to outbox, ends up in sent items, check webmail and in sent items their also but mail never gets sent.

Outlook receives mail ok, tbh havent check calendar/contacts syncing but will do today.

After installing and syncing mail Outlook didnt want to send at all, as soon as send was clicked it popped up with "Outlook is out of system resources or memory" which pointed to a MAPI problem. Have since renamed MAPI32.dll and run fixmapi.exe which resolved that issue and mail was able to be sent but this has now stopped working all together.

Wondering if its due to the account authentication process in outlook.com. Again it pretty weird that masil did send for a day or two now has stopped all together.