It's hard to say what you are doing incorrectly. The error message you show is rather yours code compilation error.
Mail.dll is compiled with and AnyCpu option on, which means it runs in native mode on both 32 and 64 bit machines.
MailBrowserControl has 2 version for 64 bit (x64 folder) and 32 bit (x86 folder).
I'm using 64bit machine and have no problems using MailBrowserControl.
Why don't you compile the x64 als true x64?
I don't undesrtand what you are referring to.
MailBrowserControl in x64 folder is compiled for x64 architecture.
MailBrowserControl in x86 folder is compiled for x86 architecture.
You can use ILSpy to verify that.
In fact you offer already a x86 version there is no reason for AnyCPU and it looks like it doesn't even work the way you compiled them.
MailBrowserControl is not compiled with AnyCpu option. It seems you're confusing two different assemblies (Mail.dll and MailBrowserControl.dll).