How to configure sendemail for no-auth?

My ~/.gitconfig looks like this, because all cloned repositories require these settings:
[sendemail]
        from = Olaf Hering <olaf@aepfle.de>
        envelopesender = olaf@aepfle.de
        chainreplyto = false
        ccover = yes
        smtpencryption = tls
        smtpdomain = sender
        smtppass = smtppass
        smtpAuth = PLAIN
        smtpserver = smtp.strato.de
        smtpuser = smtpuser
        confirm = always
        assume8bitEncoding = yes
        transferEncoding = 8bit

Now there is that one repo that requires this:

[sendemail]
        from = Olaf Hering <a@b.c>
        envelopesender = a@b.c
        smtpserver = otherhost

That "otherhost" does just plain oldstyle unencrypted SMTP.

How do I undo the global sendemail settings for that one repo? Setting the knobs to empty strings does not help: Command unknown: 'AUTH' at /usr/lib/git/git-send-email line 1455.

It seems the global smtpuser is causing the error.

Olaf