All form engines using mail notifications and mail systems are subject to the same issue. Mail delivery is not a “Webflow” issue if they tried to deliver and it and the remote server accepted it. Whether it made it to you or not, after that point is something they can’t control.
I count form fills and match them to submissions. Have not had an issue with Webflow. I am not here to defend them. I am just giving you the info.
For your information, IMAP is a client protocol to access a mailbox and act on contents. So is POP3. SMTP is used to deliver messages between different mail servers, and your client also connects to your mail providers SMTP server to send outgoing mail. If you look at your mail client settings for an account, you can see there are incoming mail settings and outgoing mail settings.
Since you mentioned sporadic deliveries to the inbox, with different providers on your end, I would confirm that no client is accessing the inbox with POP3.
If you have any clients using POP3 to access your submission mailbox, their mail clients could inadvertently be deleting downloaded messages. This is a default setting, on many mail clients with that protocol. IMAP does not delete downloaded messages when displaying them. An IMAP client just fetches the message headers, which are shown in your inbox. The complete message remains on the server unless you implicitly delete it.
Jumping to conclusions is easy, but often not helpful. Good luck.