Send email per diferente ip per email address

Hi, i was using the send email per virtual ip, but now i have a little problem, it send if i use the mail() function but now if i login via smtp it send via the principal ip address. Theres any way that users in the virtual server send mails only via virtual ip address?

i search in this web but it only says the option with psotfix 2.7 ( i ahve this one installed0 but is works via domain, not works if i login to a user

anyone? or just i have to have diferente postfix stances?