Update, just found the info which I need to modify for avast5 as it has an ini file for the mail shield:
If you want to try increasing the concurrent connections limit try this.
By editing the EmailShield.ini file using notepad, "C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\EmailShield.ini." This is for windows XP, the location will differe for Vista/Win7.
Find the [MailScanner] section and insert a new line after the section heading, and enter this MaxConnections=75 (if that line exists you can change the value to 75). Save the changes, the avast self-defence will ask if it is OK answer yes.
[MailScanner]
MaxConnections=75