Outlook app on iPhone 16 wont send emails - but receives them fine.

I feel this is a simple fix but i have been through so much support about it and I haven't had an answer.
I recently went abroad, i"m now back in the UK and have discovered that my Outlook app (important - the app) on my iphone 16 (ios 26.1) wont send emails.
It sends immediately and confirms this with a short lower third banner. It then sits in my sent folder on both my outlook app and my web outlook version, but, it never reaches the recipient. I have tested this too, by sending to my external yahoo address and sending to myself, the outlook account. No joy.

Both the web and outlook app are receiving emails externally. 

When i send an email to myself from the web version, it lands in both my web version inbox and my app version inbox. 

When i send a mail externally from the web to my yahoo personal account, it reaches it. 

When I send it from my outlook app, it doesn't. 

When i send an email from my yahoo account, from the iphone app, it reaches my outlook inbox on both my phone and the web version.

This is purely a problem sending emails from outlook app on my phone. To remind you, the sent emails show up in my sent folder on the app and the web version. Which is a madness I have never seen.

For info, all other services on my iphone appear to be working fine. I've sent and received pictures, video on whatsapp today and also used facebook messenger and uploaded photos. I've also sent a text on my iMessages to a group.

Any help with this absolute load of crazy, please let me know.

Alex