She sent a message with John to say that she couldn't come.
The above sentence is taken from Cambridge Dictionaries Online, entry: to send = to cause something to go from one place to another, especially by post.
What I don't understand is why "with" is used. Shouldn't "through" or "by" be used instead?
She sent a message through John to say that she couldn't come.
She sent a message by John to say that she couldn't come.
Is it about collocation?
I would be grateful for a reply.
Literally, for John to carry the message with him either mentally or in written form.
It implies that John was going there anyway and that her message was an additional element rather than the reason for him going there.
And 'with' would work if it was a written message - and not just John saying 'She sends her apologies'.