General
Observe Follow the notes instructions in chapter Ausgehende Kommunikation Outgoing communication (von from Connect) and as well as in the chapter Kommunikation Communication Connect -> Jira (allgemeingeneral).
HTTP Error 403 (Forbidden)
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In Connect, if the email address of an Atlassian user (as the user ) is specified instead of the user and password, username and a valid API key is specified as the password, authentication works as desired.