The client bounces the message immediately. The behavior of mail exchangers When an server accepts a message, it places a Received field at the top of the message. It does not touch any Received fields that might already be in the message. Several clients may attempt to send messages to the same SMTP server at the same time. The SMTP server ignores or rejects TCP connection attempts if it is already handling as many simultaneous TCP connections as it can.
Busy servers have to handle hundreds of simultaneous connections.
When an SMTP server pauses to think about a request , it is chewing up memory on the client host that could be used for other tasks. High-quality servers respond to every request quickly. Servers generally do not reject messages with malformed headers . Many servers do not inspect messages at all, except to count Received fields . Dumb clients Dumb clients are client hosts that do not accept mail from the network.