Mail app not sending emails with attachments on Ventura and IOS!

This is a weird problem. The mail app on ALL my Apple devices will not send emails if there's an attachment with the mail. The email just sits there in the outbox and eventually, I'll get a message asking if I want to try from a different account. I choose a different account and the same thing happens. Eventually the email may go but usually I just keep clicking 'try another account'.

The strange thing is that this also happens on my iPad mail app too.

If I download another Mail app on my Mac - I tried the outlook app, there's no issue. The mail is sent almost immediately.

If I go to the web interface of Outlook.com (my main email address), mail is sent immediately.

I would use the Outlook app but for some reason, it won't allow me to setup my iCloud email accounts..

Can anyone suggest anything?