% printf "%s\0%s\0%s" user@example.jp user@example.jp hoge | openssl base64 -e | tr -d '\n'; echo
dXNlckBleGFtcGxlLmpwAHVzZXJAZXhhbXBsZS5qcABob2dl ←コピーしておく

% telnet mx.example.jp 587
Trying 192.0.2.25...
Connected to mx.example.jp
Escape character is '^]'.
220 mx.example.jp ESMTP SENDMAIL
EHLO host.example.jp ←自分のホスト名を名乗る
250-mx.example.jp Hello host.example.jp [192.0.2.1], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE 10485760
250-DSN
250-AUTH LOGIN PLAIN CRAM-MD5 DIGEST-MD5
250-DELIVERBY
250 HELP
AUTH PLAIN dXNlckBleGFtcGxlLmpwAHVzZXJAZXhhbXBsZS5qcABob2dl ←さっきの文字列を貼り付ける
235 2.0.0 OK Authenticated ←200系の応答が返ってきたらOK
quit
221 2.0.0 mx.example.jp closing connection
Connection closed by foreign host.