Install swaks by entering the following commands in the terminal:
sudo apt update sudo apt install swaks
Description:
SMTP command-line test tool
swaks (Swiss Army Knife SMTP) is a command-line tool written in Perl for testing SMTP setups; it supports STARTTLS and SMTP AUTH (PLAIN, LOGIN, CRAM-MD5, SPA, and DIGEST-MD5). swaks allows one to stop the SMTP dialog at any stage, e.g to check RCPT TO: without actually sending a mail. . If you are spending too much time iterating "telnet foo.example 25" swaks is for you.
Homepage: http://www.jetmore.org/john/code/swaks/
Version: 20170101.0-2
Section: universe/mail