Emails from apple mail not being sent on MacBook

A periodic problem. Apple iCloud emails show in sent folder but are not being delivered. Not a problem on iPad or iPhone, only on MacBook.

It's an iCloud account using the MacBook email app, no problems about exceeding daily limit or not being connected to the internet.

The last time it happened I could send the relevant email to one other person. But when it had 20 addressees this fault happened. This was confirmed verbally by the addressees.

Not a very long email and one small document attached.

Very disconcerting as it shows as sent but hasn't. And no non delivery alerts either.

Any ideas?