We have many users who use the format

<surname>, <given name> [department]

as their sender name (preferences: "Full Name").

SOGo does not quote the "display name" [see RFC 5322: https://www.rfc-editor.org/info/rfc5322/#section-3.4] (which is probably correct because it's not required here, I guess) when sending new messages or responding to messages with such a sender name. Instead it uses the "Q encoding" as transfer-encoding [see RFC 2047: https://www.rfc-editor.org/info/rfc2047/#section-4.2].

When encoding the display name the comma is interpreted as separator of multiple addresses and since the part before the comma is not a valid email address, "@MISSING_DOMAIN" is appended here to mark the "address".

The same display error occurs when the message was sent from a different mail client which does not quote the display name.
It does not occur (in the first place) when the display name is quoted (as e.g. Thunderbird does). But it does in replies to such messages: the recipient of the reply works when sending the message, but it's wrongly displayed when opening the reply in SOGo.