The email address specified in Outlook has to be a valid email-address for the used user-account. If no match is found the default-email address of the user-account is used. What's the difference about working and non-working account?

Thanks fo r this quick reply:

1. the difference is that the working account in outlook is not based on sogo but on office365 (it is a different mailbox with different aliases).

2. in sogo, the aliases can be used to send emails, so in my opinion, the mail address and its aliases are set up correctly.

check what outlook is sending to sogo:

set SOGoEASDebugEnabled=YES and send and email with the alias
check sogo.log for this:

<SendMail xmlns="ComposeMail:">
<ClientId>...</ClientId>
<SaveInSentItems/>
<MIME>...</MIME>
</SendMail>

do a 'base64 -d' on the string between <MIME> and check the from-header

Here is a sample:

From: "Person Name" <main-address@domain.tld>
To: <mail@other-domain.tld>
Subject: Test 2
Date: Mon, 26 Nov 2018 21:20:30 +0100
Message-ID: <001101d485c5$7a7de6f0$6f79b4d0$@domain.tld>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_0012_01D485CD.DC438770"
X-Mailer: Microsoft Outlook 16.0
Content-Language: de
Thread-Index: AdSFxXjgcTIAQjQ2QTCKTOqIpOrwPw==
...

The alias does not appear here.

That would mean that Outlooks does not send correctly the information when using activesync? But does correctly when speaking to a exchange server?

Sogo can only work with infos sent in the request.
Outlook's behavior depends obviously on the used protocol.

I wonder if this is due to SOGo not implementing ActiveSync UserInformation, so Outlook is not able to verify that the user is allowed to send as the alternate address? Disclaimer: I don't use Outlook, so can't verify.

See [0004614].

I see no "<Settings><UserInformation><GET/>"-Reqeust when connecting OL via EAS.
I still think this is an OL issue.