Swiss banks might be dumb - but not as much as this... They even put oldfashioned stamps on their envelopes if requested to do so by the customers. Then, those envelopes are distributed to changing post offices so that the post print can't be attributed.

I don't doubt possible IRS agents that read this forum will like my revealings

It's different, though, if the letter is sent registered - then, the sender's name on the envelope is a must as Goldtop just explained.