When I send email from SOGo to joe@example.com, when Joe receives it, the "To:" field reads

To: "joe@example.com"@my.local.host.name

(where "my.local.host.name" is the hostname specified in my Postfix config as "myhostname").

This ONLY occurs when I send directly to a literal email address without a friendly name or angle brackets. That is,

joe@example.com -> "joe.example.com"@my.local.hostname
but
Joe Blow <joe@example.com> -> Joe Blow <joe@example.com>

Also, if I send email through the same postfix server from the iOS mail app, or from roundcube, it works fine; it's only SOGo that appears to be munging the To: field.