The current version of docker-mailserver always uses the value "mail" as DKIM selector. That works and (despite some confusion) is independent of host name. However, at least two users would like to make the DKIM selector configurable.

To do that:

• Add a DKIM_SELECTOR option with mail as the default value
• Modify the setup script to use the selector
• Modify the startup script to patch the configuration to use the selector
• Update the instructions
• Write some kind of test that proves it works