Cannot connect to multiple IMAP servers.

Hesitant to post anything about outcome since several things seemed to have worked, then disconnection again, BUT....I ran MalwareBytes, found 15 threats (including an adware problem I had with a popup yesterday). I removed them, rebooted the router, restarted and right now things are back to normal. On looking at the advanced settings, the SOCKS proxy isn't checked now. But it seemed that this happened after a previous restart, so I'm holding my breath that this sticks.....

OK, what is setting theSocks Proxy?

Some Malware or startup Item? Some Anti-Virus or VPN software?

https://www.bleepingcomputer.com/forums/t/700111/socks-proxy-keeps-re-enabling-at-every-startup/

Hi, was the Mac out of town with you?

Make a New Location, Using network locations in Mac OS X ...

http://support.apple.com/kb/HT2712


System Preferences>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.


If using Wifi, instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.

System Preferences>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

OK, good work, I think we have it nailed now! :)

Thanks. The iMac didn't go anywhere; it was just shut down for two days.

This is a weirdly intermittent problem. Sometimes clicking on the little triangle corrects the problem, sometimes not right away. All of them get online after a while, then will randomly disconnect at different times. This morning when the iMac woke, the iCloud account was offline; a few minutes later the Yahoo account went offline, then without doing anything both came back online within three minutes. Meanwhile my wifi is working perfectly, so it's not that. Before I get into the above suggestions, would this correct a problem that was not related to moving to a different location?

Well, I spoke too soon. Still having the problem after setting a new location. On the Proxy tab there is a SOCKS proxy checked. I unchecked it but it reappeared. When clicking on the triangle I still get the message "Connections to host imap.mail.yahoo.com on the default ports failed".

Short time later:

This appears to have worked. Thanks so much. I still have no idea why this happened given the computer didn't go anywhere, but whatever works!

Still holding. Thanks so much; I was about to get involved with Apple support by phone. I'm sure you saved me lots of pain.

Great news, glad I could help, thanks. :)