You can send an email using telnet it's very useful for debugging. It's something I've done several times before but because it's not something I do regularly I always forget the exact syntax so I've written this as a Note to self to remember the commands.
Finding the mail server to connect to
nslookup
> set q=mx
> gmail.com
Server: 8.8.8.8
Address: 8.8.8.8#53
Non-authoritative answer:
gmail.com mail exchanger = 5 gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail exchanger = 20 alt2.gmail-smtp-in.l.google.com.
Sending email with Telnet
$ telnet gmail-smtp-in.l.google.com 25
HELO my-reverse-dns-address.example.com
MAIL FROM: Michael@my-reverse-dns-address.example.com
RCPT TO: abuse@gmail.com
DATA
Subject: Test Email
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
molestie, fringilla turpis id, ultrices tortor,
ac consectetur massa imperdiet ac. Fusce ac porta orci.
.
if auth is required we can use base64 to work out the username and password or if base64 is not available (e.g. on windows) we can do it in python
import base64
username = base64.b64encode("michael@example.com")
password = base64.b64encode("open-sesame")
Once we have the username and password we can use them
$ telnet smtp.example.com 25
EHLO my-reverse-dns-address.example.com
AUTH LOGIN
bWljaGFlbEBleGFtcGxlLmNvbQ==
b3Blbi1zZXNhbWU=
MAIL FROM: my-reverse-dns-address.example.com
RCPT TO: michael@example.com
DATA
Subject: Test Email - Authenticated as michael@example.com
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
molestie, fringilla turpis id, ultrices tortor,
ac consectetur massa imperdiet ac. Fusce ac porta orci.
.
We can use OpenSSL to send emails using the STARTSSL command (i.e. start a standard SMTP connection then upgrade to TLS)
$ openssl s_client -starttls smtp -crlf -connect smtp.example.com:25
EHLO my-reverse-dns-address.example.com
AUTH LOGIN
bWljaGFlbEBleGFtcGxlLmNvbQ==
b3Blbi1zZXNhbWU=
MAIL FROM: my-reverse-dns-address.example.com
RCPT TO: Michael@example.com
DATA
Subject: Test Email - Authenticated as michael@example.com
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
molestie, fringilla turpis id, ultrices tortor,
ac consectetur massa imperdiet ac. Fusce ac porta orci.
.
Or We can use OpenSSL to send emails using SMTPS (i.e. start a TLS connection then do SMTP over it)
$ openssl s_client -connect smtps.example.com:465
EHLO my-reverse-dns-address.example.com
AUTH LOGIN
bWljaGFlbEBleGFtcGxlLmNvbQ==
b3Blbi1zZXNhbWU=
MAIL FROM: my-reverse-dns-address.example.com
RCPT TO: Michael@example.com
DATA
Subject: Test Email - Authenticated as michael@example.com
Lorem ipsum dolor sit amet, consectetur adipiscing elit.
molestie, fringilla turpis id, ultrices tortor,
ac consectetur massa imperdiet ac. Fusce ac porta orci.
.
Personally I prefer the idea of sending mail over SMTPS (port 465). I know it's not officially in the IANA spec, but it just makes a lot more sense to me to start an encrypted connection and then send email over it, than to start an unencrypted connection and then upgrade it to an encrypted one.
