Can't enter a Bounce Address

I’m trying to setup the Bounce Address on our site, and I found the following article:

I have edited the allow_envelope_sender to On in the file, but the Bounce Address continues to be disabled on the Settings > Workflow > Email page (I have also cleared the data cache). The note says that Server configuration may be needed. Can you tell me what would need to be changed?

Nevermind - figured it out