“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.
- 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.