It is possible to configure a HTTP based proxy for IMAP communication. This especially is important for environments which only allow connections to the internet via a proxy. Before we dive into the configuration itself it is important to understand the drawback of this approach: The used javax.mail library only supports not authenticated proxies. Therefore it is currently not possible to use a proxy with authentication.

To configure the proxy a few java system properties must be set. For example:

# IMAP
-Dmail.imap.proxy.host=proxy.local
-Dmail.imap.proxy.port=3128
-Dmail.imaps.proxy.host=proxy.local
-Dmail.imaps.proxy.port=3128
# SMTP
-Dmail.smtp.proxy.host=proxy.local
-Dmail.smtp.proxy.port=3128
-Dmail.smtps.proxy.host=proxy.local
-Dmail.smtps.proxy.port=3128
# POP3
-Dmail.pop3.proxy.host=proxy.local
-Dmail.pop3.proxy.port=8080
-Dmail.pop3s.proxy.host=proxy.local
-Dmail.pop3s.proxy.port=8080"

This will apply the proxy setting to all IMAP, SMTP and POP3 connections. In many scenarios it is advisable to exclude the primary mail account by also configuring the 'nonProxyHosts' java system properties. Please note that those properties are in opposition to the other properties ox specific properties. While "proxy.host" and "proxy.port" should be specified for both protocol variants (with and without SSL), "proxy.nonProxyHosts" must only be specified for the basic protocol.

For example:

-Dmail.imap.proxy.nonProxyHosts=my.local.imap.server
-Dmail.smtp.proxy.nonProxyHosts=my.local.smtp.server
-Dmail.pop3.proxy.nonProxyHosts=my.local.pop3.server

More complex:

-Dmail.imap.proxy.nonProxyHosts=localhost|127.0.0.0/8|[::1]|10.0.0.0/8|*.domain.com
-Dmail.smtp.proxy.nonProxyHosts=localhost|127.0.0.0/8|[::1]|10.0.0.0/8|*.domain.com
-Dmail.pop3.proxy.nonProxyHosts=localhost|127.0.0.0/8|[::1]|10.0.0.0/8|*.domain.com

This properties are applied to both secure and non-secure connections

Helm Chart

Java system properties can be configured via the core-mw chart. For example:

core-mw:
  javaOpts:
    other: "-Dmail.imap.proxy.host=proxy.local -Dmail.imap.proxy.port=3128"