A 550 5.1.1 bounce means the receiving server checked the mailbox in the address you sent to and found nothing there. RFC 3463 defines 5.1.1 as a bad destination mailbox address: the part before the @ does not match any real mailbox the receiving server currently recognizes.
This is a different failure from a domain that does not exist at all, which usually fails at the DNS lookup before the SMTP conversation even starts, and different again from a mailbox that exists but is temporarily full, which returns 4.2.2, a transient failure rather than a permanent one.
A typo at signup, or an address copied from somewhere with a character wrong, produces a 5.1.1 immediately and on every attempt afterward, since there was never a real mailbox behind it.
An employee left, a free-mail account expired from inactivity, or a subscriber closed their account. The address delivered cleanly before and fails permanently now, with no warning bounce in between.
A 5.1.1 is a list-quality problem. Suppressing the address is correct, and the real difference between a soft and a hard bounce covers which other codes are safe to treat the same way. If the bounce your list is actually returning is a 550 5.7.1 instead, the fix is in your authentication setup or your reputation, not in your list, and treating the two as the same problem wastes the fix that actually applies.
It means the mailbox the receiving server checked does not exist right now. That covers a typo, a decommissioned account, and a genuinely dead address alike, so the code alone does not distinguish them; the pattern across your list does.
A 5.1.1 is a permanent failure by RFC 3463's own definition, so a repeat offender is safe to suppress. A single occurrence on an address that has delivered cleanly before is worth a second look before suppression, in case the failure was a transient issue on the receiving side.
5.1.1 is a bad destination mailbox address, a permanent failure. 4.2.2 is a full mailbox, a persistent transient failure that is expected to clear once the recipient makes room, per RFC 3463's own class definitions.