Above warnings with trace in plack-opac-error.log In our 23.11 referred to Letters line 1558 (last line of the last catch block in sub _send_message_by_email) The caller was opac-sendbasket.pl Hard to see what happened here. opac-sendbasket will not send a mail without an email address. And Koha::Email::create checks the validity of the to address with Email::Address (no corresponding warn detected). I also saw this preceding warning: [2025/05/12 17:13:17] [WARN] Use of uninitialized value in string ne at /usr/share/koha/C4/Letters.pm line 1526. Which is the line: _update_message_to_address( $message->{'message_id'}, $email->email->header('To') ) if !$message->{to_address} || $message->{to_address} ne $email->email->header('To'); So at that point $message->{to_address} was not false! And therefore the To header must have been undefined and written back to the message! If you encounter this, should you already bail out ? Later on, Letters called send_or_die on the email (with an emptied To header). And this produced the warning "no recipients" from the module Email::Sender::Simple. Interestingly, this modules does not use Email::Address to parse the addresses (again), but uses Email::Address::XS. This triggers the above warns. Any idea?