Fix Intermittent wordpress NEX Forms Email Issue

Hi, I’m looking for a WordPress specialist to diagnose and fix intermittent email delivery from my quote request form. The issue is occurring on my staging site, tests.*****.com, which is a subdomain of my main website, ******.com. I’ve used NEX-Forms on the main site for years without these issues. On the staging site, WordPress saves the submission and shows the visitor a thank-you page, but the email notification doesn’t always arrive in Outlook. In a recent PC test, I received the notification after submitting a WebP image in Chrome. A similar test in Opera reached the thank-you page, but the email didn’t appear in Outlook. Adding WebP to the allowed file types resolved an earlier upload issue, but email delivery is still intermittent. My goal is to save every quote request and deliver its notification reliably. If a notification fails, I need a way to see the failure and recover the original request in WordPress. I’m open to keeping NEX-Forms, changing how WordPress sends email, or switching to another form solution if it’s more reliable. I’d like the fix tested on the staging subdomain before making any changes to the main site. If you have experience with WordPress forms and email delivery to Outlook, please tell me how you’d diagnose the issue and verify the fix end to end. Thanks, Mike