smtp,

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

Feb 01, 2022 · 1 min read · Post a comment
How to fix Telnet SMTP 'must issue STARTTLS command first' error

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: ignore EOF
  • -clrf: relates to the control charaters 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!
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