While conducting research for my dissertation, my collaborator and I discovered a mathematical error in published work and this error was confirmed by the author through personal correspondence. On the surface, it appears as a minor error, however it effects certain mathematical statements. The author of the work unfortunately has not published the correction in the errata for the work. It has been over a year now since our last correspondence.
The area of research I'm conducting is not widely studied so there isn't a large community of academics who are familiar with subject matter so this is unlikely to be something that will be checked by the wider community.
In the manuscript, I have written that we discovered an error and corrected it and that the correction was also confirmed by the author, and the only proof I have is email correspondence from the author. After I graduate, my email account will be shut down after a year. Therefore, would it be alright to include the correspondence I have as an appendix in the thesis in order to verify the statement I made about discovering the error and having our correction verified? The language in the email is formal and professional.