smtp, telnet,

How to fix Telnet SMTP 'must issue STARTTLS command first' error

Feb 01, 2022 · 1 min read · Post a comment

If you have already been testing the Telnet’s GMAIL SMTP, you must have seen the following error:
530 5.7.0 Must issue a STARTTLS command first. - gsmtp
This means that you have didn’t encrypt the connection first. This is quite easy to fix by following the steps below.

Prerequisites

  • Telnet
  • OpenSSL

Solution

Step 1. Run the following command:

openssl s_client -starttls smtp -ign_eof -crlf -connect <some_mail.server>:587

Parameters:

  • -ign_eof: Ignores EOF.
  • -clrf: Relates to the control characters CR (Carriage Return) and LF (Line Feed).

Step 2. To use port 465 instead of 587, run:

openssl s_client -connect <some_mail.server>:465

Conclusion

Happy Telnet email testing! Feel free to leave a comment below and if you find this tutorial useful, follow our official channel on Telegram.

smtp telnet openssl gmail mail starttls certificates security errors networking