olemus
(a) meili blokeeringu teade, enamasti spämmifiltrilt
= rejection from recipient's server to accept email,
mainly because spam filter has blocked the message

ülevaateid
https://hosteng.com/FAQFiles/SMTP%20Server%20Status%20Codes%20and%20Errors.pdf

https://www.lifewire.com/smtp-error-messages-1166422

https://serversmtp.com/smtp-error/

(b) FTP-päringu nurjumine, näiteks
otsitava faili või pääsuõiguste puudumise tõttu
= FTP access denied due to "no such file or directory" or permission problems