Swaks
swaks -a -tls -q HELO -s mail.domain.com -au [USERNAME] -ap '[PASSWORD]'
OpenSSL
openssl s_client -connect mail.domain.com:25 -starttls smtp
Telnet
Prepare credentials
$ echo -n username | base64
dXNlcm5hbWU=
$ echo -n Pa$$W0rd | base64
UGE2MzgxNzJXMHJk