WHM won't create default mail address.

[Cemtey]

Well when creating accounts WHM won't create username@userdomain.com email account automatically.
It did created them before but now it seems to be a problem. Since when user creates mail address eg someone@userdomain.com and username@userdomain.com is missing then it won't allow to send nor recieve mail with someone@userdomain.com and gives out error 500 username@userdomain.com no such user here. At webmail horde and squirellmail through cPanel port 2095.

How to make WHM automatically create default email named after created account username?

eg if username would be blackmate and domain girl.com then created default email would be blackmate@girl.com

Othervise I'd have to create default mail address to every account created by WHM manually.
Though this thing worked yesterday, now it doesn't. And I haven't touched anything.

[Infopro]

Not sure I understand you, but when you create an account;

eg if username would be blackmate and domain girl.com

blackmate is not an email account you use. You can however create an account called blackmate @ girl.com and it should work fine.

When you create an account as mentioned and setup the contact email address, all system mail for user blackmate goes to that contact email address.

[Cemtey]

No, I didn't mean that.

Just it gave with one domain email address an error that username@domain.com doesn't exist, that's why you can't send out email.
However I was logged in usernameXXX@domain.com and that email address did existed.