We host several websites on the same server using WHM but this seems to confuse the mail routing when someone has their own external mail servers - it looks locally.

We have our own email accounts hosted on the server. When creating an account for a client on the same server WHM adds the default entries to the DNS for that account. However this client has their own mail servers elsewhere. But when sending them an email it never reaches that external server - it just sees the local, incorrect one.

I realise I can update my DNS to point to the external server, but this means I am copying their settings and if they are changed, then I will also need to update mine.

Are there some settings I can use to force it to use the external servers without having to copy the settings.