If you want such chains
mail program ← Benign ← avast ← mail server
mail program ← SpamPal ← avast ← mail server

you need to force Benign and SpamPal to connect to the port, where avast is listening. Consult their documentation, it will probably be part of the username (maybe 786688#pop3.sion.com@127.0.0.1:PORT ?)