I have Kopano-Core 8.4.0-984 and also have a send as issue.
I use OpenLDAP, and have created a group with the following using phpLDAPadmin:
I log into the WebApp and update the emails “From” address to be the group’s mail address by selecting it from the GAL then add a subject and some text. I set the To address to be a local user in the system and hit send.
I get the following return email:
Unfortunately, I was unable to deliver your mail. The error given was: The user or group you try to send as could not be found. You may need to contact your e-mail administrator to solve this problem.
The following is logged in spooler.log
[info ]  Unable to open representing user in addressbook: interface not supported (80004002) [error ]  CheckSendAs(): OpenEntry failed(1) 80004002 [error ]  User "<unknown>" is not allowed to send as user or group "<unknown>". You may enable all outgoing addresses by enabling the always_send_delegates option.
If i try to send as a user with the same settings, it also fails.
Not sure whats going on here… has anyone seen this ?