Make sure your email admin allows you to send via SMTP. I have run in to this problem where I can connect through a client but not through FileMaker
Where do I have to set up this option? At the server options of my sender mail address?
When I set up an account with the same options on my Mac in Mail, sending with SMTP works.
This is an example of a working SMTP Email setting. You are welcome to try it as a template.
Have you considered that your server may not allow SMPT direct emails?
E-mail Address "Myname@me.com"
Reply-To Address: "Myname@me.com"
Outgoing SMTP Server: "smtp.me.com"
Connection encryption: None
Authentication: Plain Password
User Name: "Myname@me.com"
Password: <click to edit>
There are different port setting based on your email provider. Google (gmail) may require a app password to allow the email to be sent.
Information about using app password
smtp setting for gmail