An echo service is a bitmessage Address, that responds to messages automatically.
Depending on the implementation, this can be done 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.
As an alternative, Mailing Lists can be used to check if messages arrive (requires Subscription to it). If only receiving of the messages are needed, a broadcasting service can be used. Using Mailing Lists as test is dangerous, as most users fail to ignore messages with subjects like "TEST" or "Ignore me".