The problem with this is the probability of your server being available for the next retry is fairly low.

Usually some sort of exponential backoff is used so it might retry after 5 minutes, 15 minutes, an hour, 3 hours, 6 hours, 24 hours, 48 hours, give up.

6-8 hours is probably too much for anything serious where you don't want emails to just drop. It will work so if you're just using it to sign up to sites and stuff, you can make sure your server is on to receive the verification emails and stuff. But I wouldn't use it for anything important.

I think 8 hours starts to get into territory where they might get an informational message about the delay? That also starts to be long enough that the emails might get lost in the distant past in the client and never be seen, by the time they arrive.

I think when I used to do this, it was one advisory message every 24 hours that a message was holding in the queue, and after 5 days it would bounce, but I have to assume that those limits have shrunk in the modern day. How much, IDK; it might be worth experimenting with it though before committing to creating that situation since it might not go okay.