I used to let my beagle sniff the same mailbox for 10 minutes, then a trainer at Prospect Park told me to keep it moving | Daily Dog Walks - Babble Community
Sounds like that trainer just likes to hear himself talk, honestly. A dog sniffing a mailbox is not some big crisis, it's just a dog being a dog and taking in the world. Unless the beagle is pulling your arm off or acting wild, letting it take its time seems pretty harmless to me.