How to check the validity of mail before sending?

Question

Email

Hello.
Once upon a time, I saw a project on which, when registering, if you enter a non-existent address, then it immediately writes to you about it, and it is impossible to register.
How is it implemented? Or what are the services that provide such a service?
I found one such service on the net, but according to reviews, many letters go to spam, although they are signed by DKIM.
I would be glad for any information, and also if you recommend a service for the same implementation.

Answer 1

Reliable validity of mail is impossible to do . Because on the receiving side, I can do whatever I want with the letter. I can accept a letter to a non-existent user and send a response. I can choose not to accept an email for an existing user by sending a No such user error.
It is not difficult to technically check the entered address:
- they took the address, cut off the part after the dog
- found MX
- connected on the 25th port, said HELO / EHLO, then RCPT TO
- parsed the answer
The problem is that this answer will not be entirely reliable. Yes, it will be true on 90% of servers. But not always and not everywhere.

Answer 2

Good afternoon!
The best way to validate a list is by mailing.
Yes, there are services, there are programs. Almost all work crookedly. Suffice it to say that almost everyone can't check e-mail. And this is almost half of all Runet mail.
I use three different programs for cleaning.
The first one is simple, it quickly checks by MX records whether this domain has it or not. Non-existent domains and domains without a mail record are discarded.
The second - Email Verifier
It cleans most mails, except for Mail.Ru and some Western ones, the main thing is that it cleans Yandex and Gmail.
The third one cleans only Mail Ru - MailCheckerOnline