A family of Microsoft relational database management systems designed for ease of use.
You're using macros, so there's not much more you can do other than be sure that your Windows and Office environments are properly installed and up to date. Macros don't allow any sort of real control over the process - in essence, you're just asking Access to send the Email, and Access passes the workload off to Outlook to continue.
You should NOT have to close Outlook to do this, of course. If you do, then as mentioned above there is something amiss in your environment. Given that this process seems to have worked in the past, and assuming you have made no changes to the .accdb file (and recreated the .accde file), then environmental issues would be the likely culprit in this case.