This app was mentioned in 2 comments, with an average of 1.00 upvote
I used to use an app called eNotify to do this. You have to give it your IMAP details, but it will check the mailbox and then alert you however you like (loud, lots of times, etc).
We rely mostly on email after hours to wake us and use this app on the on call phone. It can be set to repeat your notification sound - loudly - until you manually silence it. It is also somehow more aggressive about checking email and often beats the push notification from the native mail client by a few minutes. It costs a couple bucks, so maybe only makes sense if you are sharing a phone like we are. If everyone is just carrying their own phone, it probably wouldn't work unless the company was willing to reimburse. YMMV ¯\(ツ)/¯