Collecting emails from Outlook Inbox with Delphi

Inbox folder in Outlook is not restricted to contain only mail messages. When iterating through its items you can encounter various item classes like MailItem, PostItem, MeetingItem, TaskRequestItem and many others. All of these items support different sets of properties and not all of them have Sender, SenderEmailAddress or Attachments property. If you’re interested only in mail items then you need to check item’s Class property:

const
  olMail = $0000002B;

{ ... }

for i := iNbMail downto 1 do
begin
  Mail := Inbox.Items[i];
  if Mail.Class <> olMail then
    Continue;

  { here we can assume we're working with MailItem instance }
  
end;

I doubt that Outlook ever returns null or empty item, so the check you do in your code is pointless. If you’re interested in other item classes then check out OlObjectClass enumeration.

I’m not sure why you prefer to use late-binding, because Delphi already comes with imported type library Outlook2010.pas to automate Outlook in strongly typed manner. The library is located in OCX\Servers subfolder of installation folder. In case you need to support pre-2010 Outlook versions you can use unit OutlookXP or even Outlook2000 instead. The code fore iterating mail items using the type library could look like this:

uses
  System.SysUtils, System.Variants, Winapi.ActiveX, Outlook2010;

function GetOutlookApplication: OutlookApplication;
var
  ActiveObject: IUnknown;
begin
  if Succeeded(GetActiveObject(OutlookApplication, nil, ActiveObject)) then
    Result := ActiveObject as OutlookApplication
  else
    Result := CoOutlookApplication.Create;
end;

procedure ProcessInboxItems;
var
  Outlook: OutlookApplication;
  Inbox: Folder;
  Index: Integer;
  LItems: Items;
  LMailItem: MailItem;
begin
  Outlook := GetOutlookApplication;
  Outlook.Session.Logon(EmptyParam, EmptyParam, EmptyParam, EmptyParam);

  Inbox := Outlook.Session.GetDefaultFolder(olFolderInbox);
  LItems := Inbox.Items;
  for Index := LItems.Count downto 1 do
  begin
    if Supports(LItems.Item(Index), MailItem, LMailItem) then
    begin
      { do whatever you wish with LMailItem }
    end;
  end;
end;