Also fix:
  - some docstrings.
  - variabled referenced without previous assignement.

This allows us to support encrypt mails to 'test_user@provider.com'
even if was sent to 'test_user+optional_string@provider.com'.

An email address regexp that supports all (or almost) emails is really
complex, and is_address uses a simpler one since we restrict the valid
usernames and provider names.
For that we rely on the email client to check whether the email address
is valid or not.