As the prosecution might legitimately have expected that deft would say that someone else sent the files, court's admission of uncharged emails was not unduly prejudicial; the emails were also not used to establish propensity, as the primary fact asserted was the identity of the emailer. If error, harmless, given other evidence.
http://media.ca7.uscourts.gov/cgi-bin/rssExec.pl?Submit=Display&Path=Y2018/D07-10/C:17-1311:J:Bauer:aut:T:fnOp:N:2184049:S:0
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.