IMail defaults to a limit of 30 SMTP32.EXE sending processes.
In versions before 8.0. the maximum number of Sending Processes (SMTP32.EXE) that IMail can use at once was set on the SMTP | Advanced tab. In versions of 8.0x and newer, now that the Queue Manager service controls sending, this setting is: Queue Manager | Delivery Threads.
Stop and restart both the Queue Manager and the SMTP services after a change.
Be very careful if increasing this; too many service-started processes can quickly deplete an undocumented Microsoft resource (which will cause "0xC0000142" pop-ups for each E-mail that is received afterwards, until the server crashes). For further details, see: http://www.declude.com/Articles.asp?ID=130 under the heading, Flaw #1 - Server crashing: Microsoft's Mystery Heap.
In versions before 7.1 you can change this setting in the registry. Run regedit. Go to:
If you do not find the Value named MaxQueProc you will need to create it as a new DWORD value.
If you increase this we recommend that you start with 40 or 50 (decimal). Keep an eye on machine resources. Note that it is necessary to stop and restart the SMTP service after the change. If you are running version 8, you should also stop and restart the Queue Manager service.