I don't understand. If you want the emails to arive somewhere else, what to you want with webmail access then? Because there will be no emails on the local servers to access with webmail anymore.
Or am I missing something?
You could try and setup webmail as subdomain so it can be visited like webmail.domain.com for that domain. But I don't understand why you then need local webmail for a domain which has it's email somewhere else.
Check this.
1) We'll need to add a webmail.* VirtualHost to each domain. We can make use of a feature that let's us globally specify CUSTOM# tokens, without altering the original templates.
Create the file
/usr/local/directadmin/data/templates/custom/virtual_host2.conf.CUSTOM.4.post
and add the code
</VirtualHost>
<VirtualHost |IP|:|PORT_80| |MULTI_IP|>
ServerName webmail.|DOMAIN|
ServerAdmin |ADMIN|
DocumentRoot /var/www/html/roundcube
CustomLog...