MS Access Exchange works once then fails

I am using seven workstations connected to an SBS 2007 and running MS access frontend to a MSSQL database The mail system uses Outlook on the workstations connected to the MS excahnge server wich is part of the Small Business Server. Code is written in MS Access VB to manage the DB and send emails (this is a booking agency and they send confirmations by email). On five workstations it works fine. On two workstations it works once then fails with an error message that simply indicates Access could not send the email.Workstations all run windows XP PRO (I think with same updates and revisions) Most strange and very perplexing. Any suggestions welcomed.
TIA
Sandy