Can someone tell me where the problem comes from and is there a solution ?
Do you have the same email accounts into those two users?
Is Spamcombat checking for email into those two users?
You can remove spamcombat from avast! scanning and let it only for the email program.
So, you'll get: Internet server > spamcombat > avast > email program
To do so, add the line
IgnoreProcess=spamcombat.exe
into [MailScanner] section of avast4.ini file. Check if the name of the process is really this (spamcombat.exe) into task manager.
Hope this helps.