Notes client Hanging on sending attachment with mail

Hi , We have Domino 653 on RHEL3 for 350+ users .Users access Notes client on windows XP/windows 2000 .One user is having problem sending attachments of 5 MB .His Notes client is hanging and showing server not responding ,but the mail is being sent to the recipient.The laptop which he is using is with 512 MB RAM and other users are sending attachments with no issues .What can be the issue here?

Another thing I am seeing in the Admin client is that for any use who is working if I go to the Domino Admin client >server tab >Notes users i find number of user sessions for that user.Is this a problem in the server?

kindly let us know.

Regards & Thanx,