I appreciate you for joining us here in the thread, @Anonymous. I've got your back, and I'll ensure your customers are able to receive your purchase orders in QuickBooks Online (QBO)

Before we proceed with the resolution, May I ask if you've already tried performing the recommendations given by my colleagues in this thread? If you did and still the issue persists, you can consider the other two known solutions to get through this and let your customer receive all the purchase orders you've sent to them. I'll input the steps below so  you can proceed.

You'll want to ask your customers to check their junk mail and spam folders. If not found, you can consider clearing and re-entering your email address. To begin, here's how:

1. Go to the Gear icon.
2. Under Your Company, select Account and settings
3. Select Company, then select the edit icon in the Contact info section.
4. Clear the Company email field and enter the desired address as is, even if it appears correct. (Note: Be sure not to use any extra characters or spaces before, within, or after the address you enter).
5. Select Save, then Done.
6. Send yourself a test transaction.
7. If you still don't receive the email, try using a different email address that has a different domain (for instance, if you were using a Gmail email address, try a Hotmail or Yahoo address instead).

Another suggested fix is to configure your server to accept QuickBooks Online mail server host names and IP addresses. Visit this article for more information: Solution 3: Configure server to accept QuickBooks Online mail server host names and IP addresses.

Additionally, I've got you this article in case you'd like run a purchase order report in QBO: How to run purchase order reports.

You can also check this page to learn about adding purchase orders to expenses, bills and more in QBO: Add purchase orders to expenses, bills, or cheques in QuickBooks Online.

Should you need further help managing your purchase orders in QuickBooks? Or do you need assistance performing specific tasks in QuickBooks? Know that you can always get back to me anytime. I'll be more than happy to help you once again. Take care, and have a great weekend!