Implement email validation

[vladh]

Email validation isn't fully reliable and won't necessarily guard us against spam. But it would be nice to have some kind of validation to prevent typos.

Here's what we could do:

• Check email syntax
• Verify MX records
• Verify that the SMTP server exists

Note that SMTP's RCPT TO can sometimes reveal whether a mailbox exists or not, but this is not reliable, eg it works for Gmail but not for Fastmail, so we shouldn't rely on it.

[vladh]

Or we can integrate with one of these services: https://resend.com/blog/best-email-verification-apis

[vladh]

This provider is recommended by Resend and offers a discount code: https://emailable.com/partners/resend