You can use an incoming content filter to chance it to an alternate mail host, depending upong the condition. In the incoming content filter, once a certain condition is met, you have the option to redirect that message to a different mailhost with the following action:

Send to Alternate Destination Host

Also, if it's an internal IP address, you should add that to the smtproutes.
[Network > SMTP Routes]

Hi. 

Is it possible to change the destination domain of a message and the SMTP server? For example, a message to subdomain.domain.com must be converted to domain.com, and must be relayed to altsmtphost.domain.com. With the Domain mapping feature we can change the domain, but not the routing (mails to domain.com are forwarded to smtp1.domain.com, but mails to subdomain.domain.com must be translated to domain.com and forwarded to altsmtphost.domain.com). 

Thanks in advanced.