Sure sounds like he has an A/V software running that scans POP/SMTP
conversations or gets it's hooks into Outlook.  Try disabling both of those
(since A/V software scanning the hard drive will catch all of that) and see
if it can send larger attachments fine.

In addition to Mr. Muffman's advice, increase both the server timeout value
and the send/receive interval, if one is defined.

We ran into the same problem with Outlook 2007 / 2003 and Express.  It
typically occurs with larger file attachments.  If the "server timeout" value
in Outlook is set to the default, the 'send' function well enter a repetitive
loop if the process is delayed, e.g. by a slow response from the email
server, a poor wireless connection, a large file attachment, a large distro
list, or the time required for an AV scan.

One of our users, for example, sent an email with a 4Mb attachment --
Outlook 2007 delivered 42 copies to the recipient, filling up his email
account.  Needless to say, the client wasn't happy.  Then 20 "rejected"
copies were returned to her email box -- which also filled up!

The best workaround is to set the 'server timeout' to 5+ minutes if you have
a fast connection -- or more if the connection is slow, e.g. for portables
used by salespeople, with shaky airport WiFi links.  To set this, open your
email account, pick "More Options", then the "Advanced" tab.  (Turning off AV
scans can help, but this degrades security -- and the problem can easily
recur, e.g. with a larger file attachment or a poor WiFi link.)

We have documented this bug with Outlook 2007, 2003 and Express (with
various patch levels, up to the most recent), running primarily on XP.  The
version of email server doesn't make any difference, e.g. Exchange, Unix,
ISP, etc.

I hope this is helpful.

Mark Hays