First edit your git configuration (e.g., ~/.gitconfigl.

Here’s an example for email address

name = "Joe Example"
email = ""
folder = "Inbox.Drafts"
host = imaps://
user = joe.example
sslverify = true

Then, when you want to send a git patch using runbox email, do it using:
git format-patch --to DESTINATION --stdout master | git imap-send

where “DESTINATION” is an email address to send the patch to such as “”;
you can replace “master” with a revision range.

Now log in to runbox using an email client and view the “Drafts” folder; your new patch(es) will be there.
Review them, and if you like them, send them.

Thanks to David A. Wheeler for the instructions.