pocztą pantoflową in inglese:

This agreement was reached by word of mouth, it's very informal.

We would like to advertise our products by word of mouth.

We found this hotel by word of mouth.

tłumaczenie przykładu ze słówkiem The best way to find new clients is by word of mouth.

customers often find out about reputation of a business by word of mouth.

This idea spreads by word of mouth

Such a contract needs to be written and signed. It's not enough to reach agreement by word of mouth.