A 550 5.2.0 reply means the recipient mailbox exists but is unavailable, and the server did not give a more specific reason. RFC 3463 defines X.2.0 as "Other or undefined mailbox status": the mailbox is there, yet a mailbox-level condition blocked delivery. Because it is a 5.x code, the receiver is failing the message rather than holding it.
The enhanced status code 5.2.0 is defined by RFC 3463 as "Other or undefined mailbox status": the mailbox exists, but a mailbox-level condition the server chose not to spell out stopped delivery.
What it usually points to
The mailbox is disabled, suspended, or locked by an administrator.
A forwarding or filtering rule on the mailbox failed.
The account is in a state the receiver does not map to a more precise code.
How to approach it
Because 5.2.0 is deliberately vague, read the human-readable text after the code, which often names the real reason. If the message is for a real person, ask them to check whether the account is active or over some limit. Since the leading digit is 5, retrying the same message will not help until the mailbox condition clears.
Transient sibling
The temporary form is 450 4.2.0, which asks you to retry later rather than failing outright.
Common questions
Does 550 5.2.0 mean the mailbox does not exist?→
No. The mailbox exists but is unavailable. A missing mailbox returns 5.1.1 instead.
Why is 5.2.0 so vague?→
RFC 3463 defines X.2.0 as an undefined mailbox status, a catch-all when the server has no more specific code. The reply text usually explains more.
Is 550 5.2.0 permanent?→
Yes. The leading 5 makes it a permanent failure, so the message is not retried automatically.
What is the transient version?→
450 4.2.0. It signals a temporary mailbox problem and your server retries it.