If you use multiple emails in your repos (ex. work and private) you may have encountered a situation where your commit was rejected from the remote repository, due to bad email used. In order to fix that all you need to do is to set the correct e-mail for current repository:
git config user.email "firstname.lastname@example.org"
and then, reset the author of the last commit:
git commit --amend --reset-author --no-edit
This command can also be used with
git rebase -i on edited commits.