outlook not connected to server

I am running eM Client v. 7.0.27943 with accounts for outlook.com and btinternet.com set up. It has been operating correctly on both these accounts until a couple of days ago when the outlook account intermittently shows a red triangle and the message on the e-mails says " message has not been downloaded because you are not connected to the server".  The bt account does not do this.

Please can someone help with an explanation and/or solution.