Send Email As distribution group address

I need to send a Mail from one of our distribution group. I setup the permissions for my personal account, so I can send "as" and send in the name of that mail account. Theres also an SMTP address configured for it. But when I actually try and send the message in Outlook, im getting a error back: Recipient not available, error code [0x80070005-00000000-00000000]

I tried it with internal and external addresses, but I get the same error.

Yes, we can grant the send-as permission for the mail user with cmdlet, but we cannot login his internal mailboxes, because it isn’t enabled. I also test with the external mailbox, since there is no federation ship, I got the error, [0x80070005-0x000004dc-0x00000524].

So we could know, we cannot use mail user to send as a DL in Exchange organization, even though the permission can be added successfully.