It is probably a Microsoft/Linux update that is very subtle.

It would have to be very subtle - I've not encountered any coordination between Microsoft & Linux anywhere else.

Let's just clarify - SMTP is the protocol for sending mail. POP3 and IMAP are protocols for receiving mail. Most ISPs (including Plusnet) use different servers for sending and receiving, and often different servers for POP3 & IMAP. Changing the receiving protocol from POP3 to IMAP has no effect on sending.

Various versions of Outlook & Outlook Express have had problems with sending mail. These are usually due to incorrect settings, which lead Outlook to try and send mail before receiving. This can result in wrong or missing authentication for sending and the messages just sit in the Outbox. Other than that, I know of no general problem with mail sending.

I have infrequent problems in sending mail to any address that uses a yahoo server - access is denied on the spurious basis that the server I'm using is a source of spam. The problem is actually yahoo's over-aggressive spam filtering. I just wait a few hours - my mail service usually sorts yahoo out by then.

Phil