An echo service is a bitmessage Address, that responds to messages automatically.
Depending on the implementation, this can be don in multiple ways
The service returns the complete message to the sender. There is a high risk of it being flooded with big messages.
The service returns the complete message up to a specified size, if the sent message is bigger, the rest is cut off.
The service ignores the content and only returns the message subject to the user with none or custom text.
The service answers the message without including any parts of it.