It looks like the ZennoPoster application itself uses Rebex FTP/SSL and comes with the following DLLs:
c:\Program Files (x86)\ZennoLab\ZennoPoster Demo\Progs>dir Rebex*
Volume in drive C is OS
Volume Serial Number is 6E49-0CC1
Directory of c:\Program Files (x86)\ZennoLab\ZennoPoster Demo\Progs
2017-03-03 10:13 AM 333,672 Rebex.Common.dll
2017-03-03 10:13 AM 325,480 Rebex.Ftp.dll
2017-03-03 10:13 AM 333,672 Rebex.Networking.dll
2017-03-03 10:13 AM 259,944 Rebex.Sftp.dll
4 File(s) 1,252,768 bytes
These DLLs come from Rebex FTP/SSL 2012 R3, which includes "Rebex.Common, Version=2.0.4700.0, Culture=neutral, PublicKeyToken=1c4638788972655d" and "Rebex.Networking, Version=3.0.4700.0, Culture=neutral, PublicKeyToken=1c4638788972655d".
This is most likely the cause of the issue - your plugin is looking for a new version of Rebex assemblies, but this fails because ZennoPoster already loaded an older version of the assembly.
a) Install the new version of Rebex assemblies into GAC. The resolver might be able to find them there.
b) Revert to Rebex Secure Mail 2012 R3 for your plugin. However, please be aware that this is strongly discouraged. The 4-year-old version of Rebex Secure Mail is outdated and insecure - it doesn't support TLS 1.2 and it's susceptible to Logjam attacks.
c) Modify ZennoPoster to make it possible for plugin authors to reference whatever assemblies they want. Possibly by loading plugins into separate AppDomain, or by using AppDomain.AssemblyResolve event. I won't get into details because this would have to be done by ZennoPoster vendor, but see this article for more information.