Why smtp server can not be telnet?

Question

https://www.webdigi.co.uk/blog/2009/how-to-check-if-an-email-address-exists-without-sending-an-email/

I am following the above instruction to telnet an email server of gmail. But I got an error. Could anybody let me know what is wrong? Thanks.

$ nslookup -q=mx gmail.com
Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
gmail.com   mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com   mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
gmail.com   mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com   mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com   mail exchanger = 5 gmail-smtp-in.l.google.com.

Authoritative answers can be found from:

$  telnet gmail-smtp-in.l.google.com. 25
Trying 64.233.168.27...
Trying 2607:f8b0:4003:c07::1a...
telnet: Unable to connect to remote host: Network is unreachable

Firewall blocking port 25 and that blog post is really old. That method is unreliable at best — Jacob Evans, Oct 04 '18 at 03:55
Are you able to telnet any other smtp server? E. g.: telnet smtp.mail.com 25 — Vadim, Oct 04 '18 at 06:59

score 0 · Answer 1 · answered Oct 04 '18 at 04:27

To use port 25 you need to use a relay service for google apps - unless you want to only be able to send to other Gmail users - which requires setup in the admin portal if you are on Google apps not just consumer Gmail.

If you're just trying to test connection/confirm email without a mail app try port 465 or 587.

Google apps smtp guide

Why smtp server can not be telnet?

1 Answers1