Egyelőre - végre(!) - innen egy visszajelzést kaptam a sikertelen elküldésről:

 Hi. This is the qmail-send program at citromail.hu.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<sapirico@protonmail.hu>:
161.97.88.211 does not like recipient.
Remote host said: 550 5.1.1 <sapirico@protonmail.hu>: Recipient address rejected: User unknown in virtual mailbox table

Még próbálkozom más módokon...