Reverse proxy a remote IMAP/SMTP server with NGINX

Question

Is it possible to reverse proxy a remote (managed by another company, think Google Apps or Office 365) SMTP/IMAP server through NGINX? Example:

I host example.com, and use mailhost.com to host my mail. I tried adding 2 CNAME records on example.com to accomplish this, but it didn't work. [smtp.example.com -> smtp.mailhost.com, etc]

Preferably, I want one subdomain (mail.example.com), pointing to both smtp.mailhost.com and imap.mailhost.com, differentiating between the two using ports. [mail.example.com:993 -> imap.mailhost.com:993, smtp.example.com:587 -> smtp.mailhost.com:587]

Is this possible? The only reason I want to do it is to make setting up mail clients easier for my website's users, and to stop confusion when asking why I am asking them to connect to a seemingly random domain to get emails. I understand this may cause some problems with DKIM etc.

Is this task easy, more hassle than it is worth or impossible? I'm capable of setting up a mail server, but would rather use a ready solution.

Answer 1

In my opinion the NGINX mail proxy server guide lacks of one very important information that was necessary for me to get something similar working:

Using a PHP Script on an Apache Server as the IMAP Auth Backend

I spent some weeks to find a working solution as mocking the auth service was not working properly for me. After disabling all SSL features and adding the auth php script I was finally able to receive emails with IMAP via the NGINX proxy.

One Problem that still exists for SMTP is: NGINX doesn't forward the auth command to the server: https://forum.nginx.org/read.php?2,58181,58186#msg-58186.

Therefore disabling auth for SMTP is necessary to make use of NGINX mail proxy. If someone finds a workaround please post the solution here: https://stackoverflow.com/questions/56031592/nginx-forwarding-smtp-auth-credentials-to-next-server/56583571#56583571

Here are some related links to the topic I want to share:

• Nginx as reverse SMTP proxy
• reverse proxy for multiple SMTP / IMAP servers
• IMAP: Reverse Proxy?
• Configuring NGINX as a mail proxy server with load balancing based on user
• Nginx TCP forwarding based on hostname

Answer 2

This is pretty well covered in the Configuring NGINX as a Mail Proxy Server guide in the official docs.

You should note though that you might experience issues with mail deliverability unless you are able to make any SPF, SSL, and DKIM configuration match what's upstream.