Your email could be bouncing back to you for a variety of reasons. Here are just a few:
- The email address you are sending to is no good. You might get a error, "unknown user"
- The email address you are sending to is full, and will not receive new messages.
- It has blocked the email for anti-spamming purposes.
- (Most common)Their email server is too busy to accept email.
We have been getting a lot of complaints on reason #4. Error looks something like:
Delivery failed, X attempts [email address]
To further explain: This is because the destination mail server is too busy to receive your mail. Our mail server tries X times, and then will bounce the email back to you. Sorry, we can not force their mail server to accept your email.
But, I can send them email fine from my other email account with another provider.
You may have happened to email them at a time when their email server was NOT too busy. Spam and email viruses are at an all time high, and are over 10 times what they were just a year ago. Many mail servers, including Microsoft, hotmail, etc, are overburdened with having to handle 10 times as much load, and can be busy, that sending mail servers can simply NOT get an email through to them.