I have 3 emails accounts I have used regularly for the past 3 years, 2 pop mail that use SMTP servers and one IMAP (Outlook.com). All of the mail accounts can receive emails without an issue but one of the POP mail accounts has decided that it can't send any more, the outgoing email just sits in the outbox until eventually the OS gives up. The reason why I say OS rather than specify is because this behaviour started happening on 4 devices at the same (iMac and MacBook Pro both running El Capitan and iPhone 6 and iPad Pro, all of which are fully updated). I have spent hours trying every combination of settings I could until I had to leave to go and visit the folks so I took my iPad and iPhone with me so I could carry on messing about with it and to my surprise both devices sent emails seamlessly while connected to their wireless network with the setting I knew should work but as soon as I got back home and connected to my wireless network (Time Capsule) they stopped again. To test I disabled the wireless connection on both devices and they sent just fine again via 4G. It would now seem that the issue is the wireless configuration here but what I don't understand is that the Time Capsule has been used for the past 4 months without issue and I haven't changed anything on it? The only thing I can add is that 2 days ago I had to reinstall the OS on the iMac after it wouldn't boot after running a Clean my Mac scan (never again!) but the mail was definitely working before then but surely that wouldn't affect the other 3 devices too?