maybe provide us with some details on what you have already tried doing
to get git-send-email working, the exact error you're getting, etc, so
that others will be better able to help you.
i know in some cases mac users may have to run:
sudo -H cpan Net::SMTP::SSL IO::Socket::SSL
to install missing dependencies.
also, your smtp config may have to have have tls enabled:
git config --global sendemail.smtpEncryption "tls"
your mailserver may not accept plain password authentication, in which
case you'll have to use an app specific password or oauth.
but without further info on what you've already tried and what error you
are getting is like shooting in the dark.