My iMac suddently stopped sending emails. Any ideas?

Thank you for your reply and my apologies for the delay in getting back. I tried several self-help solutions including the one that you recommended. I spoke to my email service provider but they were of limited help and advised that I talk to Apple Help. This I did using their 0800 number and after several attempts using different approaches I was advised to wipe my hard drive and back up from a period immediately before my problem started. This seemed extreme to me but they are the experts! In the event my email service is back to normal but I still have no idea why I could not send emails. Not sure that there is a lesson here for anyone else, but I share it anyway.