code cleanup
Begin abstracting the X509 certificate database logic
Abstracted X509CertificateDatabase
Fixed copyright
Fixed InternetAddressList.Parse() to not throw incorrectly
Updated MessageIdList to not wrap the Message-Ids in <> to be consistent with other code
Added unit tests for InternetAddressList.Parse() in addition to TryParse()
Fixed MessageIdList.ToString() to wrap references in angle brackets
Added the ability to clone a MessageIdList
Added more explicit casts to/from System.Net.Mail equivalents
Add the MailMessage.Body text as the first part in the multipart/alternative
Drop support for casting MimeMessage to System.Net.Mail.MailMessage

There's no way to make this work reliably because System.Net.Mail
is very limited in the types of MIME structures and MIME parts
that it supports.
Improved System.Net.Mail.MailMessage -> MimeMessage conversion logic
When converting from MailMessage to MimeMessage, copy over the Priority as well
Set the 'type' parameter on the multipart/related
Use a more appropriate internal .ctor
Fixed unit tests
Bumped version to 0.26.0.0
Added reference to rfc2557
Disambiguate the Dispose method in the docs