Just in case this saves someone the considerable hassle I went through ..
If you run a project on Sourceforge and use their mailing lists you may have been tempted recently to switch to another provider, like google groups, whether due to considerable SPAM arriving at SF or regular flake-outs with 3-5 days no mails delivered.
A standard Mailman install has an export feature which is all you need to get a text output of all the email addresses subscribed to your SF list – but the SF version doens’t have this feature, perhaps it’s an older release.
Instead of copying hundreds of records, one copy/paste for each letter in the alphabet from SF Mailman admin page, you can actually go to SF -> your project -> Mail -> Admin -> Administer/Update Lists and you get exactly the text export you need.
I have always considered SF text link navigation some of the most unintuitive available on the web, but this is the first time it has lost me a significant amount of time.