Problem

Blank spaces and backslash are not supported in the MailGate admin password. While you will be able to set a password that contains a blank space or backslash and login with it, MailGate will not allow for a cluster to be joined should such symbol is used in the password string.

You will get the following error:

[!] Invalid Primary Appliance password. (503)

Resolution

You need to change the admin password so that it does not contain any blank spaces or backslashes.