I have actually been looking for the same thing. I have created a work around.
Please dont use this as a permanent soulution, but if you have couple of factors in order
1. All your settings are done correctly. (serverport, outgoing smtp port , username and password, email adress)
2. internet is connected.
your mail will 99,5% of the time be sent.
So I just created a custom dialog box that said message sent.
I know is not the correct way, but if your factors are correct this will give the user a safer feeling of the message beening sent.
If you want to go into more details, I just learned. Just in the very moment i write this message.
I just looked up in help in filemaker pro advanced.
If you add an calculation Get (last error) look at the following error messages then you can create your result
1501 SMTP authentication failed
1502 Connection refused by SMTP server
1503 Error with SSL
1504 SMTP server requires the connection to be encrypted
1505 Specified authentication is not supported by SMTP server
1506 Email(s) could not be sent successfully
1507 Unable to log in to the SMTP server
I hope that helps
PS: I just seached for SMTP in help.
Thanks for leading me to the right solution. I created a script with an If statement that cycled thru all of the SMTP errors, and if none apply, will generate a Custom Message stating that the message was sent successfully.