offering webmail to customers on primary domain by deactivated mail-services

[almightya]

hello!

i want to offer my customers on my primary domain the webmail service.
(https://webmail.primarydomain.xxx, http://webmail.primarydomain.xxx)

this works fine, if i enable the mail service on this domain.
BUT the problem is, that i use external mail-accounts (hosted exchange) and not the pop3/imap-
accounts on the plesk server. so if any customer on the plesk server (other mail-domains on the plesk server)
send a mail to my hosted exchange mail-address the mail can´t be delivered, because the server want to deliver this
on the local (plesk) server. so i can´t get mails from other mail-users on this server or web-formulars and so on.

how can i offer my customers the webmail service on my primary domain WITHOUT enable the mail-feature on this.
hope you can help me.

thanks!