I take it there's no debug option when you get the error?

You could step through the code using F8.

Breakpoints (F9) can also be set up to halt the code at certain places.

You can combine the two.

For example set up a breakpoint on a line before a section of 'suspicious' code.

Then run the code to that point as normal and step through the next section of code.

One place it might be worth looking at is the sub 'Mail_workbook_Outlook' where presumably you are automating Outlook.