Get new Registration EMails 3 times and in a loop

Hi all,
i have setup a new discourse forum. I use an external Mailprovider, which works for my other mailboxes well. If i register a new user i got the registration mail 3 times. After some time i got again the registration mail 3 times and so on.
My settings are ok, i make a console test and can connect to the mailserver end send a test mail.
I rebuild the app, but i have still the same problem.
Any idea or do you need further information?

i looked in Sidekick and found:

Jobs::HandledExceptionWrapper: Wrapped Net::ReadTimeout: Net::ReadTimeout with #TCPSocket:(closed)

But i didnt find a solution. Seems Mail is sending, but not recognized by discourse. Any idea?
In another post i found the option open_timeout and read_timeout, where can i change the values?