I have set up an email alias and set the option "Allow target to send mail using this alias as origin". For my understanding this means that I can send mails with this address as sender address. Have I understood this correctly? If so, how do I have to configure the mail client so that this happens? If I want to authenticate myself at the ispconfig-mailserver with the alias address and the destination mailbox password, this does not work. Which login data do I have to use to be able to send mails in the alias name? Thanks for your answers!
Thanks for this quick and useful hint. But if i do so as you mean: Thunderbird means that this credentials are already used by an existing mailbox. Ok - so I have to use two mailuseragents.
No, you have to add this as an alias address in Thunderbird too, and not as a separate mailbox. In german Thunderbird translation, its called "Weitere Identitäten". Thunderbird then uses the same smtp out settings for this alias then automatically.