Large mailing: items sit in send box

Windows 2000
Outlook XP Professional: Outlook
We do not have an inhouse exchange server
Email provided by ISP

One of my users is sending out an email to something like 1000 people. Here is what he did:

He opened a new email
Copied and pasted his email message
Copied and pasted a subject
From various documents (Word, email, excel, web sites, etc.) he copied email addresses. Anywhere from 1 to 300 or more addresses and pasted them in the BCC field.
He left the To: field blank

None of the addresses were saved to Contacts or the Address book.

(Yes, I know there is a better way to do this.)

Anyway, he created many of these emails in the above manner and then sent them.

After a long time, he found them all sitting in his Outbox folder. He would click on "send and receive" but could not force them to be sent.

He said he had to delete them all and then start the process over. This time as soon as he created a message he would send it. However, he would still get emails hung up in his email box that only had, say, four email addresses in the BCC field.

The message was not graphics intense nor did it have attachments.

Any idea what might have been his problem?