Over quota?

Hello Fred.

Are you aware that you can adjust the amount of server storage space allocated between email and your iDisk storage with your .Mac account and if so, how must storage space do you have allocated to email only?

Are you storing sent messages on the server or locally on the hard drive with the Mail application which is available under the Mailbox Behaviors tab for your .Mac account preferences?

This error message sounds like it came from the recipient's incoming mail server so does this recipient have a .Mac account - an @mac.com email address?

If not, the problem is on your end.

Since the recipient is a .Mac member as well, this problem is on the recipient's end.

His/her server storage space allocated to email has been exceeded.

The term "Quota" is synonymous with "mailbox full" and is the more common term for IMAP accounts which a .Mac type account is.

When receiving such a return email error message after a message was successfully sent by the SMTP server used to send messages with the account, 99% of the time the error message is sent by the recipient's incoming mail server.