Попробуйте сохранить:
String savepath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) + @"\" + filename + ".msg";
mailItem.SaveAs(savepath);
FileInfo mailItemToCompress= new FileInfo(savepath);
Сжать:
public void Compress(FileInfo mailItemToCompress, string compressedMailItem)
{
using (FileStream originalFileStream = mailItemToCompress.OpenRead())
{
using (FileStream compressedMailItem= File.Create(compressedMailItem)
{
using (GZipStream compressionStream = new GZipStream(compressedFileStream, CompressionMode.Compress))
{
originalFileStream.CopyTo(compressionStream);
}
}
}
}
Затем прикрепите свой mailItem:
mailItem.Attachments.Add(compressedMailItem);