mail app won't send

On my 2017 MacBook Pro, emails sent by my iCloud account stay in the outbox, although the message at the bottom of the column says they are in the process of being sent, "Message Sending". After a while a warning message appears to “p25-imap.mail.me.com", and a suggestion I try again. Trying again makes no difference. In mail preferences, Account information status shows the account online (green dot, although sometimes it seems to get stuck on "checking" for some time). Server settings shows the outgoing mail account to be to be iCloud.

I'm connected to Virgin cable by ethernet, and have no problem opening web pages in Safari.

I also have a Mac mini and an iPhone which have no problem sending emails.

I'm running Mojave 10.14.6. I haven't upgraded to Catalina as I've read bad reports of it and also I have some 32bit software I need to use.

Look forward to hearing from you.

Norman