Validate email address

[matthiaswh]

https://documentation.mailgun.com/en/latest/api-email-validation.html#email-validation

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Email validation with Mailgun requires a monthly plan (i.e. not pay as you go). It is probably good enough to have a built in validator that checks against a regex.

It might also be helpful to have a 'likely invalid' check that looks for misspellings of common email providers / TLDs. E.g. there are a fair number of @gmaol.com and @example.con that would pass validation, but are clearly typos.

Add a public mail.validate() method that can be called prior to attempting to send an email.