A step-by-step guide to send email with postfix via Yahoo mail.

I am posting the (hopefully) complete guide to send email from Yahoo mail, as requested.
This guide is slightly different from what you need if using Gmail, as shown here, and possibly other servers as well, but this specificity is crucial to make it work.

Note: I have actually tested it, and successfully received email in my destination address.
It also works well with local (outgoing, incoming) addresses.

The two answers by Raffa (1, 2) and further comments helped me a lot.
But there was still significant reading and tinkering needed to make it work.

1. Install postfix and other packages.

   
   

   $ sudo apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules
   
   

   
   



2. The configuration post-install will show up. If not, use $ sudo dpkg-reconfigure postfix. For the successive screens, use: 1. Satellite system (but perhaps irrelevant since you will modify the postfix configuration file later), 2. myubuntubox, 3. [smtp.mail.yahoo.com]:465, 4. root, 5 and following: leave the defaults.

   
   



3. Enable your Yahoo account to interact with postfix, with an App password. This is a one-time password, which avoids leaving the Yahoo mail password in your PC.
   Go to https://login.yahoo.com/account/security , Account Security -> App Password and generate a password.
   You will have to use it later on.
   This works for sure if not using 2-step verification, and possibly with 2SV as well.
   
   
   Note that in Gmail "App Passwords can only be used with accounts that have 2-Step Verification turned on." (source).
   In this case, you will use the App password since postfix, as many other apps, is a "less secure app".

   
   



4. Add the following lines to /etc/postfix/main.cf

   
   

   relayhost = [smtp.mail.yahoo.com]:465
   
   smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd_yahoo
   
   smtp_sasl_security_options = noanonymous
   
   smtp_sasl_auth_enable = yes
   
   smtp_use_tls = yes
   
   smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
   
   smtp_tls_wrappermode = yes
   
   smtp_tls_security_level = encrypt
   
   smtp_generic_maps = regexp:/etc/postfix/regex_map_yahoo
   
   

   
   

   If present in /etc/postfix/main.cf, comment line

   
   

   smtp_tls_security_level=may
   
   

   
   

   with an initial hash mark # to avoid a warning message.
   I am not posting the complete file. If you find issues with these instructions, please post feedback.

   
   



5. Create/edit /etc/postfix/sasl_passwd_yahoo so it contains

   
   

   [smtp.mail.yahoo.com]:465    [email protected]:<password>
   
   

   
   

   <password> should be replaced by the generated App password.

   
   



6. Use the following commands

   
   

   $ sudo chmod 0600 /etc/postfix/sasl_passwd_yahoo
   
   $ sudo postmap /etc/postfix/sasl_passwd_yahoo
   
   $ sudo touch /etc/postfix/regex_map_yahoo
   
   $ sudo chmod 0600 /etc/postfix/regex_map_yahoo
   
   $ sudo nano /etc/postfix/regex_map_yahoo
   
   <---- Creating/editing the file ---->
   
   $ sudo postmap /etc/postfix/regex_map_yahoo
   
   $ sudo systemctl restart postfix.service
   
   

   
   

   When creating/editing /etc/postfix/regex_map_yahoo, its contents should be

   
   

   /.+@myubuntubox/   [email protected]
   
   

   
   



7. Send email with, e.g.

   
   

   $ echo "This is the body of an encrypted email" | mail -s "This is the subject line" [email protected]