I have webmail.sitea.com and webmail.siteb.com configured as DNS lookups - that works ok.

I have tried having multiple servers in the nginx servername directive (webmail.sitea.com webmail.siteb.com) and new clients coming in on "http(colon slash slash)webmail.site?.com" get redirected to https(colon slash slash)webmail.site?.com/mail and presented with the login screen - so far so good

On entering a successful username and password, I get a "cannot find server" and the url that is trying to be loaded is "https(colon slash)mail/"

the redirect line in the config on the external NGINX is "return 301 https(colon slash slash)$server_name$request_uri"

I have just tried creating separate NGINX sites for each webmail.site? instance, and it does the same thing

However, if I use https(colon slash slash)webmail.site?.com it has no problems at all

Any ideas?