In addition to what Atropos7 said (which is correct n every way!), you could be running up against a situation where even though the domain and webmail sub-domain has propagated, the mailbox is not yet created.
I have no idea why (possibly because mailbox creation is queued and "batch processed"?), but, when creating new mailboxes, it can sometimes take over 24 hours for the mailbox to be functional.
An indication that this is the situation might be if you log-in to webmail.yourdomain.tld and receive an error message that indicates your mail cannot bet displayed.
In every case where this has happened to me, the mailbox does get created eventually - it just seems to be the last thing to go "live". Good Luck!