Suppose that plato.example.com is my mail server.

I have the following files:

• plato.example.com.key, created on my linode (along with plato.example.com.csr which is submitted to Namecheap when purchasing Comodo PositiveSSL)
• Comodo gives me back these files: AddTrustExternalCARoot.crt, PositiveSSLCA2.crt, and plato_example_com.crt

What is the proper way of setting up Postfix and Dovecot using all these files?

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I am a bit confused because on Nginx (if plato.example.com is going to be a web site), I have to cat three files together:

cat plato_example_com.crt PositiveSSLCA2.crt AddTrustExternalCARoot.crt > plato.example.com.crt

Then I set it up like this:

ssl_certificate /etc/whatever/plato.example.com.crt;
ssl_certificate_key /etc/whatever/plato.example.com.key;

Do I have to cat files together in a similar way for Postfix and Dovecot?

Yes, you do it exactly the same way as nginx. (In my experience, most server programs work that way, although I have encountered a few that require the intermediate/chain certs[1] to go in a separate file.)

Relevant documentation for Dovecot and Postfix: http://wiki.dovecot.org/SSL/DovecotConf ... rtificates and http://www.postfix.org/postconf.5.html# ... _cert_file

[1] plato_example_com.crt is your certificate, and the others are known as "intermediate" or "chain" certificates.