Reject email addresses that known to be undeliverable during signup.
Some users signup with invalid email addresses, which causes the welcome
email (which contains the email confirmation link) to bounce. Too many
bounces hurt our ability to send mail.
We check that an email address is undeliverable by checking if the
domain has a mail server and if the server returns an invalid address
error when attempting to send mail. This isn't foolproof since some
servers don't return an error if the address doesn't exist. If the
checks fail we know the address is bad, but if the checks pass that
doesn't guarantee the address is good. However, this is still good
enough to filter out bad addresses for popular providers like Gmail and
Microsoft that do return nonexistent address errors.
The address existence check requires being able to connect to mail
servers over port 25. This may fail if your network blocks port 25,
which many home ISPs and hosting providers do by default.
* Move emails from users table to email_addresses table.
* Validate that addresses are formatted correctly and are unique across
users. Existing invalid emails are grandfathered in.
* Add is_verified flag (the address has been confirmed by the user).
* Add is_deliverable flag (an undeliverable address is an address that bounces).
* Normalize addresses to prevent registering multiple accounts with the
same email address (using tricks like Gmail's plus addressing).
