How to relay incoming mails which asked to specail domain to the

For example, I have two mail hosts( A and B) use sendmail as MTU.
[A] host can delivery mails to gmail, but can't delivery mails to
hotmail.
[B] host can delivery mails to hotmail, but can't delivery mails to
gmail.

So I want to [A] host relay any incoming mails ( To: [at] gmail.com )
which going to delivery to gmail to [B] host, [B] host delivery these
mail to gmail.

[B] host relay any incoming mails ( To: [at] hotmail.com ) which going to
delivery to gmail to [A] host, [A] host delivery these mail to
hotmail.

How to config sendmail