How safe is this? I don’t know much about email security. Can the ISP intercept and edit the email message?
A bigger concern might be Russian-monitored email servers, especially given that around 70% of the market share is occupied by Russian providers [2], and email isn't considered personal data in Russia [3].
[1]: https://transparencyreport.google.com/safer-email?hl=en
[2]: https://help.returnpath.com/hc/en-us/articles/115001108411-C...
[3]: https://www.plesner.com/insights/articles/2016/01/data-local...
Update: as for transit, see adam-p's comment.
Of course, if your ISP is also your email provider (get a new email provider), then your ISP will be able to see and modify your email.
We also sign our apps and provide detailed instructions for verifying the authenticity of them: https://psiphon.ca/en/faq.html#verify-psiphon-authentic (You can't get to that if you can't get to the internet, but it's hard to do much better. We can't email the instructions, since then the instructions could also be modified. But it's entirely possible for some users to be able to load a webpage but not download a 6MB binary.)