Address validation via server lookup

The recipients mail address should not only be verified by correct format of the mail address, furtheron it could be checked, if the address really exists.

Here is a simple example, which uses telnet to communicate to the mailserver
https://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/

## EDIT
It seems to make no sense to do the validation on the client side, it would make more sense to implement a webservice, which also buffers the valid addresses