Non-encrypted email in transit is a concern, but AFAIK most users should already be protected nowadays. (Google estimates [1] put it at around 9/10, tho probably not representative of Russia)

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.