What steps will reproduce the problem?
1. Send stanzas frequently to domains that don't exist
What is the expected output?
After a certain amount of time has passed without making a successful connection to a domain, new attempts to contact that domain should be limited.
What do you see instead?
Prosody will immediately attempt new connections to domains that have just failed to connect.
On servers with a lot of traffic this leads to Prosody almost persistently having pending connections to domains that don't exist (and most cases never will, due to typos, etc.)