Swaks
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).
example
swaks --server smtp.endpoint.com:2525 \
--auth-user smtpuser \
--auth-password smtppass \
--to to@example.com \
--from from@exmple.com \
--header "Subject: SMTP from swaks" \
--add-header "Content-Type: text/html" \
--body 'This is a test body. <a href="https://google.com">go to google</a>'