The best way (and easiest, if you find the installer) to access
nsf files is to install Lotus Notes client. It's a great file format, database like, capable of storing any kind of data (mail is just a part of Notes/Domino).
I've worked on it up until v7 for a company I used to work for back then. It's really straight forward, you install Notes app and just open the .nsf file as you'd open an .xls file in Excel. In the past, IBM allowed a trial edition to be downloaded using a free IBM account. Nowadays the product has been sold to another company, I don't know if they still supply a free trial version. If not, perhaps you can locate on the Internet older IBM trial versions which can be obtained legally. Since you are an Ubuntu user, there were officially supported Ubuntu Linux versions available. I used Notes before these Linux versions were available (v7-8 shipped with Linux versions also, if I recall well), previous versions played nicely using Wine. So even if you obtain a v5 Notes client for example, you can use it through Wine.
PS: I don't think that's a retro question, Domino/Notes is still produced and supported :)