The MTA normally rewrites addresses as it enqueues messages to its channel queues. No additional rewriting is performed during message dequeue. This presents a potential problem when host names change while there are messages in the channel queues still addressed to the old name.
The connectalias keyword tells the MTA to deliver to whatever host is listed in the recipient address. This is the default. The keyword connectcanonical tells the MTA to connect to the host alias for the system that to which the MTA would be connected.