| `250` | 250 2.1.5 OK: Recipient address accepted | `valid` | Safe to Send. The recipient mailbox was explicitly verified as active and deliverable by the target server. |
| `251` | 251 User not local; will forward to <user@other> | `valid` | Safe to Send. Mailbox is accepted on an alias or forwarding node. |
| `421` | 421 Service not available, closing transmission channel | `unknown` | Temporary Deferral. Remote server is under heavy load or throttling our probe. 0 credits charged. |
| `450` | 450 4.2.0 Mailbox unavailable: busy or greylisted | `unknown` | Greylisted. MTA implements greylisting requiring repeat delivery attempts. 0 credits charged. |
| `451` | 451 4.3.0 Local error in processing | `unknown` | Temporary Error. Remote server experienced an internal queue exception. 0 credits charged. |
| `452` | 452 4.2.2 Insufficient system storage / Mailbox full | `risky` / `unknown` | Mailbox Over Quota. Recipient's inbox is completely full and temporarily rejecting new mail. |
| `500 / 501` | 500 Syntax error, command unrecognized | `invalid` | Malformed Command. Target server rejected the address syntax. |
| `550` | 550 5.1.1 User unknown / Mailbox does not exist | `invalid` | Dead Address (Hard Bounce). Remove immediately from sending lists to protect sender reputation. |
| `551` | 551 User not local; please try <forward-path> | `invalid` | Relay Denied. Server refuses to accept incoming mail for this recipient without explicit authentication. |
| `552` | 552 Exceeded storage allocation | `invalid` | Permanent Quota Overflow. Abandoned mailbox exceeding storage thresholds permanently. |
| `553` | 553 Requested action not taken: mailbox name not allowed | `invalid` | Policy Block. Recipient name violates server administrative naming rules. |
| `554` | 554 Transaction failed / Client host rejected | `invalid` / `unknown` | Access Denied. Remote MTA policy block or aggressive anti-spam reputation firewall. |