尝试以下示例代码Easily Retrieve Email Information from .EML Files

CreateItemFromTemplate只适用于MSG/OFT文件。对于EML文件，您需要在代码中显式解析该文件或使用第三方库（如Redemption - I am its author）：
以下代码将创建一个MSG文件，并使用Redemption（RDOSession对象）将一个EML文件导入其中：

set Session = CreateObject("Redemption.RDOSession")
  Session.MAPIOBJECT = outlookApp.Session.MAPIOBJECT
  set Msg = Session.CreateMessageFromMsgFile("C:\Temp\temp.msg")
  Msg.Import "C:\Temp\test.eml", 1024
  Msg.Save
  MsgBox Msg.Subject

然后，您可以使用邮件（RDOMail）访问它的各种属性（主题、正文等）

为了从.eml文件创建MailItem，您可以使用以下两个步骤：首先打开一个Outlook进程示例，然后使用Outlook API创建MailItem。

string file = @"C:\TestEML\EmlMail.eml";
  System.Diagnostics.Process.Start(file);
  Outlook.Application POfficeApp = (Outlook.Application)Marshal.GetActiveObject("Outlook.Application");  // note that it returns an exception if Outlook is not running
  Outlook.MailItem POfficeItem = (Outlook.MailItem)POfficeApp.ActiveInspector().CurrentItem; // now pOfficeItem is the COM object that represents your .eml file

虽然Outlook可以打开EML文件，有没有办法做到这一点编程只与VBA。所以我创建了这个VBA宏循环通过一些文件夹，并打开每个EML文件使用***SHELL EXEC***。它可能需要几毫秒，直到Outlook打开EML文件，所以VBA等待，直到在ActiveInspector中打开的东西。最后，这封电子邮件被复制到一些选定的文件夹，并且（在成功的情况下）删除原始EML文件。
在这里查看我的完整答案（和代码）：https://stackoverflow.com/a/33761441/3606250

假设已安装outlook...
仅对于msg文件，当outlook正在运行/已打开时，您可以使用OpenSharedItem

Microsoft.Office.Interop.Outlook.Application appOutlook = new Microsoft.Office.Interop.Outlook.Application();
var myMailItem = appOutlook.Session.OpenSharedItem(strPathToSavedEmailFile) as Microsoft.Office.Interop.Outlook.MailItem;

对于eml文件或msg文件（Outlook将打开并弹出）：

var strPathToSavedEmailFile=@"C:\temp\mail.eml";
//Microsoft.Win32 namespace to get path from registry
var strPathToOutlook=Registry.LocalMachine.OpenSubKey(@"Software\Microsoft\Windows\CurrentVersion\App Paths\outlook.exe").GetValue("").ToString();
//var strPathToOutlook=@"C:\Program Files\Microsoft Office\root\Office16\OUTLOOK.EXE";

string strOutlookArgs;
if(Path.GetExtension(strPathToSavedEmailFile)==".eml")
{
  strOutlookArgs =  @"/eml "+strPathToSavedEmailFile;  // eml is an undocumented outlook switch to open .eml files
}else
    {
     strOutlookArgs =   @"/f "+strPathToSavedEmailFile;
    }

Process p = new System.Diagnostics.Process();
p.StartInfo = new ProcessStartInfo()
{
    CreateNoWindow = false,
    FileName = strPathToOutlook, 
    Arguments = strOutlookArgs
};
p.Start();

//Wait for Outlook to open the file
Task.Delay(TimeSpan.FromSeconds(5)).GetAwaiter().GetResult();

Microsoft.Office.Interop.Outlook.Application appOutlook = new Microsoft.Office.Interop.Outlook.Application();
//Microsoft.Office.Interop.Outlook.MailItem myMailItem  = (Microsoft.Office.Interop.Outlook.MailItem)appOutlook.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);
var myMailItem = (Microsoft.Office.Interop.Outlook.MailItem)appOutlook.ActiveInspector().CurrentItem;

//Get the Email Address of the Sender from the Mailitem 
string strSenderEmail=string.Empty;
if(myMailItem.SenderEmailType == "EX"){
    strSenderEmail=myMailItem.Sender.GetExchangeUser().PrimarySmtpAddress;
}else{
    strSenderEmail=myMailItem.SenderEmailAddress;
}

//Get the Email Addresses of the To, CC, and BCC recipients from the Mailitem   
var strToAddresses = string.Empty;
var strCcAddresses= string.Empty;
var strBccAddresses = string.Empty;
foreach(Microsoft.Office.Interop.Outlook.Recipient recip in myMailItem.Recipients)
    {
    const string PR_SMTP_ADDRESS = @"http://schemas.microsoft.com/mapi/proptag/0x39FE001E";
    Microsoft.Office.Interop.Outlook.PropertyAccessor pa = recip.PropertyAccessor; 
    string eAddress = pa.GetProperty(PR_SMTP_ADDRESS).ToString(); 
    if(recip.Type ==1)
        {
        if(strToAddresses == string.Empty)
            {
            strToAddresses = eAddress;
            }else
                {
                strToAddresses = strToAddresses +","+eAddress;
                }
        };
    if(recip.Type ==2)
        {
        if(strCcAddresses == string.Empty)
            {
            strCcAddresses = eAddress;
            }else
                {
                strCcAddresses = strCcAddresses +","+eAddress;
                }       
        };
    if(recip.Type ==3)
        {
        if(strBccAddresses == string.Empty)
            {
            strBccAddresses = eAddress;
            }else
                {
                strBccAddresses = strBccAddresses +","+eAddress;
                }       
        };
    }
Console.WriteLine(strToAddresses);
Console.WriteLine(strCcAddresses);
Console.WriteLine(strBccAddresses);
foreach(Microsoft.Office.Interop.Outlook.Attachment mailAttachment in myMailItem.Attachments){
    Console.WriteLine(mailAttachment.FileName);
}
Console.WriteLine(myMailItem.Subject);
Console.WriteLine(myMailItem.Body);