Hello. Sometimes it happens that somebody forget to update her PGP key
with some new email address, so to send her messages I need to do it on
an old or not appropriated address… how can I do that?

Also, I just noticed, trying to write to an address like
<user+something@domain.tld>, that it doesn’t recognize the address with
the + sign (which is usually used on most mail servers to send messages
to the same address but helping other’s MUA to sort mails), so I’m
forced to write to <user@domain.tld> without the plus sign to encrypt…