perl -MMIME::Base64 -e 'print encode_base64("username\0username\0password");'

fill in user (x2) and passwd , cut & paste the output string

telnet to the smtp host:

type

auth plain [paste]

S: 220 mail.example.com ESMTP Postfix (1.1.7)
C: EHLO example.com
S: 250-mail.example.com
S: 250-PIPELINING
S: 250-SIZE 10240000
S: 250-VRFY
S: 250-ETRN
S: 250-AUTH DIGEST-MD5 CRAM-MD5 GSSAPI PLAIN LOGIN
S: 250-AUTH=DIGEST-MD5 CRAM-MD5 GSSAPI PLAIN LOGIN
S: 250-XVERP
S: 250 8BITMIME
C: AUTH PLAIN dGVzdAB0ZXN0AHRlc3RwYXNz
S: 235 Authentication successful
C: QUIT
S: 221 Bye