then set up postfix and send the messages using mailsend, there might be a sender included in postfix too, but I'm not sure.
Mailsend could be all you need, and skip postfix completely.
Since sending emails via your provider can be tricky, some block outgoing SMPT traffic, I actually use a service called Pushed for sending messages to an app, implementation is fairly simple - you just call an URL using curl.
curl -X POST -s --form-string "app_key=234234wdffsdfsdfsf" --form-string "app_secret=lsjflsjdfof790wr78lsjdfs7dfsldfjsdfr34" --form-string "target_type=channel" --form-string "target_alias=ZZZZZZZ" --form-string "content=my message text here" https://api.pushed.co/1/push
there are other web services providing the same solution, but this was fairly easy to set up, and
had less restrictions to amount of free messages sent on a monthly basis.
then try to connect, make it fail, and adjust the search signature accordingly.
problem is there might be several error messages, for a failed logon, so you'll have to watch the log,
or trigger on everything, but successful logons.