Failed to send share with mail

Nextcloud version (eg, 12.0.2): 16.0.1
Operating system and version (eg, Ubuntu 17.04): Ubuntu 18.04 Server LTS
Apache or nginx version (eg, Apache 2.4.25): nginx 1.17.0
PHP version (eg, 7.1): 7.3.6

The output of your Nextcloud log in Admin > Logging:

Swift_TransportException: Expected response code 354 but got code “503”, with message "503 5.5.2 Need rcpt command

Hello, if we try to share a folder / file with an external contact by email, we get the message: Failed to send share by mail.
The internal e-mail notification works perfectly.
In the e-mail server settings we use our internal Exchange server.
Does the sharing notification even use the email settings in Nextcloud’s settings? Or is there a separate mail server for this function?
Greetings and thanks.