He was saying that you expect the mail man (postman - UK) to deliver your mail, you entrust him with your message and you expect him to mail it, not to write his own letters. Similarly,
"Its not our own mail we're delivering, Its God's mail for his people ...You can watch the message here:
its not whatever we want to talk about ...
he's given us the message of his word, and he asks us to deliver that to his people."