The best regular expression i've came across till now is the following:
r'([a-z0-9][-a-z0-9_+.][a-z0-9])@([a-z0-9][-a-z0-9.][a-z0-9].(com|net)|([0-9]{1,3}.{3}[0-9]{1,3}))'
this approach relies on adding every domain name you want your user to be able to log in with.
i just added com and net since they are the most popular ones but you can simply add more
