Local unset override global options

I have default settings for sending email in the global git config. However, for one repo different email settings are necessary. That specific SMTP server does not want authentication, therefore smtpuser should be unset.

The issue is that global smtpuser is set for the default SMTP server and unsetting local smtpuser results in the global smtpuser being inherited. What I'd like is smtpuser to be unset for this particular repo.

Is there any way to "unset override" options locally? If not, is this something that could and should be implemented?

Stefan