What does this SMTP error mean?
SMTP 551 User not local appears during mail sending or delivery. In practice, check the full bounce text, sender and recipient address, SMTP authentication, port, encryption, MX, SPF, DKIM, DMARC, IP/domain reputation and anti-spam filtering.
What should be checked in Hostilla?
For a Hostilla mailbox, check the password, SMTP port, STARTTLS/SSL, mailbox quota, SpamBox, delivery logs, mail DNS records and possible CSF/LFD blocks after repeated bad logins. For website forms or WordPress, check SMTP plugin configuration, sending limit and sender address in the same domain.
How to prepare a support request?
Send Hostilla.pl support the full SMTP 551 response, sender address, recipient address, attempt time, mail program or application name, domain and whether the problem affects one message, many recipients or all mail. Affordable good mail hosting also means correct DNS, SPF, DKIM, DMARC, backups, mail moving, PHP-FPM, OPcache, Redis or Memcached for mail-sending applications, clear price in PLN and renewal for the next year.