I hope you are right!
Yes, that option is available but I haven't selected it. In fact I added a declaration in configuration to say using 4 not 6.
I will test it now before I go into other things I was thinking like IP tables firewall blocks or the position of the MailDir and maybe other users set up previously causing confusion. The other one is in destinations where I saw in a tutorial the localhost and domain inputed but I though as had already been declared in main.cf file that $domain and $hostname would follow in sync for that.
Thanks and will report back.