diff --git a/MimeKit/Tnef/TnefPart.cs b/MimeKit/Tnef/TnefPart.cs
index 8bd968d944..d4fcd2b3ad 100644
--- a/MimeKit/Tnef/TnefPart.cs
+++ b/MimeKit/Tnef/TnefPart.cs
@@ -404,7 +404,8 @@ static void ExtractAttachments (TnefReader reader, BodyBuilder builder)
 							attachment.ContentBase = prop.ReadValueAsUri ();
 							break;
 						case TnefPropertyId.AttachContentId:
-							attachment.ContentId = prop.ReadValueAsString ();
+							if (MailboxAddress.TryParse (prop.ReadValueAsString (), out MailboxAddress mailbox))
+								attachment.ContentId = mailbox.Address;
 							break;
 						case TnefPropertyId.AttachDisposition:
 							text = prop.ReadValueAsString ();