This is most likely because of obfuscation.
We already send you a link to Mail.dll which was obfuscated with relaxed settings - this should help.
You need to add following dependencies from nuget:
System.Security.Cryptography.Algorithms (>= 4.3.1)
System.Security.Cryptography.Pkcs (>= 4.5.0)
System.Security.Cryptography.X509Certificates (>= 4.3.2)
System.Security.Cryptography.Xml (>= 4.5.0)
System.Text.Encoding.CodePages (>= 4.5.0)