If the purpose of the message is good, so the messenger becomes a better person, and if the purpose of the message is bad, the messenger is (chas v'shalom!) similarly effected.
This is one possible explanation, he says, of how the tzaddikim that were sent to explore the land of Israel were able to fall so far in such a short time, only forty days.
To me this has interesting implications in so many fields. But, it becomes immediately obvious how much this applies to watching one's speech. When our messages are negative, that has a negative impact both on the speaker and on the listener. There isn't even the buffer of messengers in the case of the spoken word. If a messenger is so thoroughly affected in only forty days, how much more so the composer of the message?