Nigel, the last time I had this happen it took weeks to chase down the real solution. Hope you have a solution quicker than that! Anyway, thought I would post my solution in case it can help you with this very similar sounding issue you are suddenly having. In the end it turned out to be due to changes on my ISP's end. It may be worth your while to contact your email provider's customer service to see if there are any changes impacting outgoing mail.

Thanks for your response, E04Wellness. It took me less than weeks... With the help of my daughter, who is a professional programmer, we tracked down a URL showing that one of the SMTP settings had an "il" added, for Israel. I spent about an hour talking to support at Netvision/Cellcom in Israel who had no idea how to help me. Fact was, it was THEY who made the change, but just left customers in the lurch. God, I hate when that happens... Anyway, crisis averted, back to normal.

WOW! So glad you've solve it! Those types of issues are so frustrating.