“Passer comme une lettre à la poste” is a French expression that literally means to pass or go through like a letter at the post office. English meanings include the following:
- to go off without a hitch
- to work perfectly
- smooth sailing
According to expressio.fr, a French-to-French translation is to pass or go through easily, without incident. According to the site, in the 19th century the expression often applied to the idea of digesting food easily.
example sentence:
- Les prétextes que le président a donnés pour ses erreurs étaient des mensonges, mais ils sont passés comme une lettre à la poste. The excuses that the president gave for his mistakes were lies but they worked perfectly.
