On my mac in mail I get a "!" in a black triangle??

[#1]

I have asked the Apple comunity why this happens and after carrying out instructions they gave me I get a message:- Connections to the server “mail.plus.net” on the default ports timed out.
Is there a problem with the Plusnet server or what settings should I have on my mac???
NB this has only just started to happen.

[#2]

You're almost certainly suffering from the problem highlighted in this thread,  see reply #3 and reply #12 for fixes.

[#3]

I am not that confident a user to go messing about in "library" can any Plusnet staff give me the settings for my mac so I can delete the Plusnet mail and reload it??? My problem is only with Plusnet both my sms & iCloud e-mail are fine???

[#4]

You don't need to fiddle around with the plist file. It can all be done within the mail account settings.
See post #17 for the settings you need to check.
(Untick "Automatically detect and maintain account settings". Untick "Use SSL" and lastly tick "Allow insecure authentication".)