One of the questions I often get from users of Mail.app results from a less-than-descriptive error message: “Cannot send message using server XXX”. It suggests a problem with your mail server when, in reality, the problem is with the message itself.
The next time you see an error message like that, press the “Edit Message” button, then check all the addresses in the To, CC and BCC fields. If the address is in a blue bubble, click on it to see which address is checked. If you find one that doesn’t look right (no “@some.domain” or similar problem), correct it and try sending again. For example, if you are typing quickly, you might inadvertently address a message to “fred” when you meant to send it to “email@example.com”.
If the addresses look right, check the size of your attachments. As a general rule, your attachments shouldn’t exceed 5 MB.
Can’t see an issue so far? Go to your web browser and make sure you actually have a connection to the Internet. Sometimes you will get disconnected for some reason or another, and Mail doesn’t warn you of that.
Finally, call us and we’ll be happy to troubleshoot.