Find out smtp maximum email size limit

To find out smtp maximum email size limit, connect to smtp server with telnet:

telnet mail.example.com 587

or with openssl when need SSL support:

openssl s_client -crlf -connect smtp.gmail.com:465

Next send SMTP command:

EHLO smtp.example.com

In example with google smtp server:

EHLO smtp.gmail.com

Result is:

250-mx.google.com at your service
250-SIZE 35882577
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH XOAUTH2 PLAIN-CLIENTTOKEN
250-ENHANCEDSTATUSCODES
250 CHUNKING

SIZE is maximum email size in bytes.