Just to further elaborate on that...

If you used a OSI-approved licensed program it would be impossible to link it to your own code without also open sourcing your code.  Using it externally would be acceptable (e.g. calling it via exec() or via the command line), but because you would be distributing it with your program you would be required to provide the source code for the zip program at users' request.  Linking to the authors site would not satisfy this requirement, you would need to provide a link to download the source from your own server.  I doubt this would be a big problem though, most zip programs are quite small and I doubt many people would be interested in downloading it from David's servers anyways.

An option to zip attachments

1) when selecting attachment: attach and zip

2) when attachment is already selected, see size, zip, see new size...

3) when there are several attachments, zip them all wrapped, zip selected wrapped

Now, for unzipping, I hesitated a moment, thinking that the Os's default unzipper should do it. But:

1) It is a utility and not a document format viewer provided by a vendor

2) It is very close to attachment management

3) Part of the programming work is already done by adding zip function

So I think Pmail could provide me a way of opening a zip folder, to see which files are in there, which files I want to extract...

Yes it'd be a great thing to have and would absolutely fit in with the attachment handling overhaul we've discussed, but I think that the licensing issues involved would make it unlikely.

What would be the licensing problem?  There are a plethora of open source zip/unzip programs that could be used.  An OS license would be a problem if you intended to compile or link a library into your own code, however, if you just use it as an external program (similar to how Spamhalter uses SQLite) then you'd have no issues at all.

Great! I just assumed that David would have to pay to include any third-party ZIP handling code. I would seriously doubt that he's interested in writing his own.