There are two settings that can make the dialog appear. The first place is a setting in the preferences dialog. To access the preferences dialog, go to the Edit menu (on Windows) or the FileMaker menu (on Mac), Preferences, Plug-ins tab, double-click SMTPit Pro, Extras tab, and the After Send tab. Make sure the "Popup 'Email Sent Successfully" dialog check box is "unchecked".

The other place it could be is in the SMTPit_Send function. Take a look at the function:

SMTPit_Send ( GetTranscript ; ShowResultDialog )

The first parameter is for a transcript which means if it is set to True, then the function will return the transcript of interaction between the plug-in and the mail server. The second parameter is the result dialog parameter. If set to True, it will show the Email Sent Successfully dialog. Check the second parameter of the function to make sure it is set to False. For example:

SMTPit_Send ( "True" ; "False" )

If there is a field name in place of the 2nd parameter like this:

SMTPit_Send ( "True" ; DB::ShowResult )

Then check that field and change the field data to False. Alternatively, you can also just remove the parameter if you never want to use it. Like this:

SMTPit_Send( "True" )