When an add-in wants to save an e-mail by calling the Microsoft.Office.Interop.Outlook.MailItem.SaveAs Method on an e-mail that is encrypted by using GpgOL an exception is thrown.
The SaveAs method works fine for normal and encrypted (S-MIME direcly by Outlook- not by the GpgOL) e-mails.
In my test add-in I simply handled the selection changed event for saving the currently selected e-mails to disk (temporarily). The issue occured on Windows10 x64 and Office 2013, 2016 and 2019. Everything with latest updates.
Here is the exception:
System.Runtime.InteropServices.COMException (0x80004004): Vorgang abgebrochen (Ausnahme von HRESULT: 0x80004004 (E_ABORT)) bei Microsoft.Office.Interop.Outlook._MailItem.SaveAs(String Path, Object Type) bei OutlookAddIn1.ThisAddIn.SelectionChanged() in C:\Users\msc\Documents\Visual Studio\Projects\OutlookAddIn1\OutlookAddIn1\ThisAddIn.cs:Zeile 32.
You can find the simple example Outlook Add-In attached.
Can you confirm that?