After specify tcp::collect, the SMTP services is not working

First, understand that a single TCP packet will never hold anywhere near 4MB. It'll be spread across many TCP packets. The length value in the TCP::collect command is there, reasonably, to collect a specific amount of data versus the whole packet payload, and can certainly speed things up if what you're looking for is early on in the packet. In lieu of that, TCP::collect without a length argument, and someone correct me if I'm wrong here, should only cache the payload of the first packet in a session, unless the TCP::collect is called again immediately after a TCP::release. I've run into very few situations where the length value was absolutely required, and if you're experiencing an actual memory leak I'd open a support case.