Setting up SMTP on top of Postfix for 2 websites on the same server

You can configure Postfix for multiple domains by using a hash file, which contains the list of domains used for local delivery. Look at the file /etc/postfix/main.cf for this section:

# Define the domain list as hash file or as list in the config file.
virtual_alias_domains = hash:/etc/postfix/virtual_domains

Then, create the file you specified in the main.cf, and add your domains:

example.com   #domain
example2.com   #domain
Test1.com #domain

Save the file, then run this command to create your hash db file:

postmap /etc/postfix/virtual_domains

Finally, reload Postfix: /etc/init.d/postfix reload

The Postfix documentation may help you further if you want to make a more complex setup: http://www.postfix.org/VIRTUAL_README.html

Can you provide a bit more information on your use-case? It is possible to use domain mapping to handle email for multiple domains in postfix and this guide will help you in setting that up. SASL will then let you configure authentication.

What I am confused by is your reference to SendGrid. Are you relaying emails from postfix via sendgrid rather than sending them directly? Is this being used to send emails from a web site or script or do you have users who will connect via SMTP directly? I think by better understanding your intended use case I can help you to find a good solution.