Mail accounts dropping out

Hi,
I Have two email accounts on my iMac, one iCloud and one BigPond (my ISP). Both of these accounts seem to be dropping their connection somehow. They get the small exclamation mark next to their inbox name which when clicked on asks me to take it back online but when I try, it doesn't reconnect. Quitting Mail and reopening usually puts both accounts back online and messages are able to arrive and leave again... until the next time they drop out (within a day or two). The two accounts dont drop out at the same time always, one may still be online whilst the other is not...

Does anyone have any ideas on how to fix this? Or at least how to figure out why the accounts are dropping out?

Thanks.

(I still have an internet connection when the accounts are offline. Both accounts are also set up on an ipad on the same wifi network and that has no issues with connection.)