This looks like a mismatched .net version referenced.
Make sure you reference Mail.dll via nuget:
https://www.nuget.org/packages/Mail.dll/
-or-
When referencing directly make sure appropriate assemblies are also referenced:
System.Security.Cryptography.Algorithms (>= 4.3.1)
System.Security.Cryptography.Pkcs (>= 8.0.0)
System.Security.Cryptography.X509Certificates (>= 4.3.2)
System.Security.Cryptography.Xml (>= 8.0.0)
System.Text.Encoding.CodePages (>= 8.0.0)
for .net framework 2.0-4.8 no additional references are needed.
Can you please send us the eml file (preferably zipped)?
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