Is your Exchange or ISP mailbox full? Are you sending an attachment that
exceeds your allowable limit? Sender is you.
What's the exact error message? That would be helpful.

Don't describe or paraphrase the professed NDR (non-delivery report)
e-mail.  Copy and paste it in your post (but munge out the e-mail
address, yours and their's), and include the headers since that shows
who actually sent the NDR.  Provided details unless you want vague
replies.  Read the following:

http://66.39.69.143/goodpost.htm
http://support.microsoft.com/kb/555375
http://users.tpg.com.au/bzyhjr/liszt.html

Could be you have been spamming or bulk mailing and used up your own
quota with whomever is your UNNAMED e-mail provider.  Many that
provide PERSONAL accounts will limit:

- The size of each e-mail.  That is its TOTAL size as seen by the
e-mail provider, not the filesize for whatever file(s) you happen to
attach to an e-mail.  ALL e-mail, and I mean ALL, gets sent as
plain-text.  Encoding is used for HTML and also for attachments.
Encoding binary to text to include in a MIME part within the body of
your e-mail will enlarge the size of the attached binary file by 2 to
3 times its original size.

- The number of recipients per e-mail.

- How many mail sessions you can have in, say, 5 minutes.

- How many messages you can send per day.

- How many recipients (in total across all messages) to whom you can
send e-mails.  You might be able to send 500 messages per day but
perhaps you are also limited to 500 recipients, so if 100 messages had
5 recipients in each then you hit your maximum recipient quota.

Check with your e-mail provider on their anti-spam/anti-abuse
policies.  They'll know why they generate the message that you got.
The rest of us here can only guess.