feat: add email validation function to lower bounce rates

[cstockton]

The goal is to only return an error when we have a very high confidence the email won't be deliverable.

This is currently going to be added as a draft for the team to review. I haven't actually implemented any paths that call this or configuration around when it is activated.

[coveralls]

Pull Request Test Coverage Report for Build 12189005080

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 158 of 217 (72.81%) changed or added relevant lines in 7 files are covered.
• 84 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.3%) to 57.268%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/conf/configuration.go	13	15	86.67%
internal/mailer/noop.go	3	5	60.0%
internal/mailer/mailme.go	0	5	0.0%
internal/api/mail.go	10	17	58.82%
internal/mailer/mailer.go	3	13	23.08%
internal/mailer/validate.go	119	152	78.29%

Files with Coverage Reduction	New Missed Lines	%
main.go	10	0.0%
cmd/migrate_cmd.go	16	0.0%
internal/api/external.go	58	75.23%

Totals
Change from base Build 11932304745:	0.3%
Covered Lines:	9696
Relevant Lines:	16931

---

💛 - Coveralls

[kangmingtay]

as discussed internally, one suggestion would be to add this validation in the sendEmail function (

auth/internal/api/mail.go

Line 603 in 7de6bfb

func (a *API) sendEmail(r *http.Request, tx *storage.Connection, u *models.User, emailActionType, otp, otpNew, tokenHashWithPrefix string) error {

)

[kangmingtay]

in terms of where we should include this validation, i'm thinking it can either be used in:

1. The validateEmail API struct method (

   auth/internal/api/mail.go

   Line 563 in 49fbbf0

   func (a *API) validateEmail(email string) (string, error) {

   )
2. Or in sendEmail, before we make the request to the SMTP server (

   auth/internal/api/mail.go

   Line 603 in 49fbbf0

   func (a *API) sendEmail(r *http.Request, tx *storage.Connection, u *models.User, emailActionType, otp, otpNew, tokenHashWithPrefix string) error {

   )

[kangmingtay]

left a bunch of readability nits that can be cleaned up in a follow up