How to send a mail directly to SMTP server without authentication?

Thats a real good question, and i am replying inline.

I would like to send an email directly from a script to a Gmail email account, by connecting directly to smtp.gmail.com.

First of all smtp.gmail.com is not a mailserver which accepts mail (from other mailservers), but rather allow Gmail users to login and hence send or check email. If we want to find out the Gmail mailservers that accepts mails from other mailservers. We can run the following cmd on the terminal:

dig mx gmail.com +short

output:

10 alt1.gmail-smtp-in.l.google.com.40 alt4.gmail-smtp-in.l.google.com.5 gmail-smtp-in.l.google.com.30 alt3.gmail-smtp-in.l.google.com.20 alt2.gmail-smtp-in.l.google.com. 

Since gmail-smtp-in.l.google.com. has the lowest value of 5 we use it as the preferred mailserver

However, I would prefer not to have the gmail password in the script. From what I have read, it appears that Gmail requires authentication before it will deliver any mail, including to its own users.

Exactly one uses smtp.gmail.com to login and send/check emails to/from their respective accounts, therefore we require user credentials. However we don't need credentials on sending emails to its mail server i.e gmail-smtp-in.l.google.com (Example below)

My question is, how is mail coming from another SMTP server ever delivered, since that SMTP server will not have Gmail credentials. Does Gmail only require authentication for "anonymous" senders, and since my script is running on a personal computer, it is subject to higher security? Here is the python script I am running:

As I have made myself clear from discussion above we don't need Gmail credentials to connect to Gmail mail servers, however if we connect to Gmail mail servers using personal computers we can get away with sending a few emails, but to send more emails we need to build domain reputation and accountability using DKIM, SPF etc (Thats a whole different spectrum).

The following python script sends email to a gmail account without authentication.

import smtplibfromaddr = 'sending@example.com'toaddrs  = ['receiving@gmail.com']# string inside msg below must have "Subject: <subject line>\n"# for a subject to be sent, and "To: " for the recipient to be shown in the emailmsg = '''To: receiving@gmail.com    Subject: Subject line here\n    The body goes here    .'''msg = msg.format(fromaddr =fromaddr, toaddr = toaddrs[0])# The actual mail sendserver = smtplib.SMTP('gmail-smtp-in.l.google.com:25')server.starttls()server.ehlo("example.com")server.mail(fromaddr)server.rcpt(toaddrs[0])server.data(msg)server.quit()  

Or try the following Telnet snippet

telnet gmail-smtp-in.l.google.com 25HELO sendingdomain.comMAIL FROM:<user@sending.com>RCPT TO:<playingwithtelnet@gmail.com>DATAFrom: <user@sending.com>To: <playingwithtelnet@gmail.com>Subject: Just a test emailThe body of the mail goes here.          .QUIT

You can use some external SMTP servers without authentication (or a local SMTP), but the sent message will be caught by Google's spam filter because the msg["From"] is @google.com, while the actual SMTP is not smtp.gmail.com.

Those SMTP servers also must have correct reverse zone in ISP's DNS and otherwise this smtp will be blocked by google.

You can't use gmail smtp-server without authentication. It's a google policy. You need to enter your account password. But there is another way. You can use GAE (Google App Engine) with Gmail API. In this way you can send messages directly.