Bouncing Messages

[troothteller]

With version 2.4 on Windows 7 64-Bit, emails to friends I have had for at least three years all of a sudden bounce. They come with the response "Temporary failure" in the subject line until the last one shows "Permanent failure." Troubleshooting shows that it happens through SeaMonkey, my default email client. This morning, with technical support on my telephone line, I have used my ISP's website to send emails to those addresses and received no error messages. The domain of both addresses, belonging to a husband and wife, are ptd.net. PTD stands for PenTeleData. The husband, who has worked in computer services, assures me that there is no problem at that end. What problem can occur at my end?