Is the configuration this?
Hosting company hosts website for DomainA.com
Hosting company is outside your network.
You run Brightmail and have DomainA.com as local domain ( you accept mail for them).
Does the hosting company send the e-mail for the user (e.g. like visiting a news site and clicking the e-mail this to a friend)? or is the link just a html mailto: link?
In the 1st case, you might want to add the IP address of the Hosting company's mail servers to you trusted senders list.
In the 2nd case, this is just like any other user sending an e-mail to user@domainA.com - that they get the e-mail address from a contact-us button doesn't matter.