Mail.dll requires all encodings to work (you may be processing emails created in any language).
On Xamarin platform, please include all encodings by setting MandroidI18n property in Android .csproj file:
<MandroidI18n>All</MandroidI18n>
-or-
Go to:
Android project -> Properties -> Android Options
and select all possible encodings in:
Linker properties -> Additional supported encodings
Reference:
https://www.limilabs.com/blog/mail-dll-add-mandroidi18n-in-xamarin-release-mode