Mail App loses connectivity after waking from sleep

Mail app has worked perfectly since first using it. For the last few days however, I have noticed when I wake the mac from sleep the mail app is not working correctly. All acounts have a warning triangle and are behaving as if they were taken offline. Using the take online option, or online status or even checking for new mail fails. Nothing will make the app function again. The only way to get the app to put accounts back online and check for new mail is to quit and relaunch.

I am really sure this has online been in the last few days, perhaps a week or so. Prior to this, the Mail app worked perfectly.

I have not installed any new software, or made any changes to the OS. Any ideas what might have caused this change of behaviour?

Many thanks.

Shirley