Forum OpenACS Q&A: Re: Bad sender address syntax - postfix
It looks to me, as if in your installation, the bounce prefix (parameter EnvelopePrefix of package acs-mail-lite) is not set correctly (look under /acs-admin, you can see/change the parameters of the installed services there). The parameter should have a value like e.g. "bounce".
The versions of OpenACS and the installed packages are listed by the package manager (/acs-admin/apm/). For OpenACS, this looks currently like the image below.
Hope this helps
I had actually left the EnvelopePrefix blank, and your answer led me there.