Topic: Positive pay file message type

I am trying to determine the best message time to send un-posted transactions from a customers QuickBooks to my financial institution (where I am a developer) to be integrated with our positive pay technology.  Payment messages seem to be strictly related to bill pay and most others I have investigated were designed for information retrieval only.

Before I decide to use the mail message to send the information (and capture it on the back end), is there a better message type to use for this sort of thing?  Even a generic type would suffice for my needs.

Thank you,