That's a pretty dumb way to validate an email address. You can just end the connection after a successful RCPT TO and no one needs to be the wiser. The method causes a false validation in the case of an intermediate mail relay that accepts everything but that is such a bad idea that no one does that.
#> swaks --quit-after RCPT --TO kdskr3j2@gmail.com
=== Trying gmail-smtp-in.l.google.com:25...
=== Connected to gmail-smtp-in.l.google.com.
<- 220 mx.google.com ESMTP a199si3828494itd.133 - gsmtp
-> EHLO example.com
<- 250-mx.google.com at your service, [xx.xx.xx.xx]
<- 250-SIZE 157286400
<- 250-8BITMIME
<- 250-STARTTLS
<- 250-ENHANCEDSTATUSCODES
<- 250-PIPELINING
<- 250-CHUNKING
<- 250 SMTPUTF8
-> MAIL FROM:<bob@example.com>
<- 250 2.1.0 OK a199si3828494itd.133 - gsmtp
-> RCPT TO:<kdskr3j2@gmail.com>
<** 550-5.1.1 The email account that you tried to reach does not exist. Please try
<** 550-5.1.1 double-checking the recipient's email address for typos or
<** 550-5.1.1 unnecessary spaces. Learn more at
<** 550 5.1.1 https://support.google.com/mail/?p=NoSuchUser a199si3828494itd.133 - gsmtp
-> QUIT
<- 221 2.0.0 closing connection a199si3828494itd.133 - gsmtp
=== Connection closed with remote host.