TL;DR
Titan Email documented 5.4.6 for mailbox and domain quotas. It permanently ended that attempt, but does not by itself prove the address is invalid.
What this code means
X.4.6 has the RFC sample “Routing loop detected,” but the IANA mirror confirms no class. This page is published on exact Titan quota responses, not as a registry-confirmed universal meaning.
Provider examples
550 5.4.6 <user@example.com>: Recipient address rejected: Recipient Quota Exceeded;550 5.4.6 <user@example.com>: Recipient address rejected: Mailbox Does Not Exist : Domains Hourly Quota Exceeded;550 5.4.6 <user@example.com>: Recipient address rejected: Mailbox Does Not Exist : Domains Daily Quota Exceeded;Technical meaning
The three accepted Titan responses name recipient quota exceeded, domain hourly quota exceeded, or domain daily quota exceeded. They document that provider's quota use of 550 5.4.6; they do not establish that all 5.4.6 replies mean the same thing.
Delivery status
The 550 class permanently rejected the recorded attempt. A later submission may be a new attempt after a confirmed quota or account change; it is not an automatic retry of the same transaction.
- Class
- Permanent failure
- Retry
- Do not retry unchanged
- Suppression
- Check the full context
Retry decision
Stop automatic retries of the unchanged attempt. For a mailbox quota, seek confirmation that the mailbox condition changed; for domain quotas, wait for the relevant window or obtain administrator confirmation before a new attempt.
Suppression decision
Do not suppress an address from 5.4.6 alone. The evidence includes shared domain quotas, and even provider response wording is not independent proof of address validity.
Common causes
- A Titan recipient mailbox quota was exceeded.
- A Titan-hosted recipient domain exceeded its hourly or daily quota.
Diagnostic steps
- Preserve the exact response and identify its mailbox, hourly-domain, or daily-domain wording.
- Correlate attempts by recipient domain and time to distinguish a shared domain condition from a mailbox condition.
- Ask the recipient administrator or provider to confirm the applicable quota before a new submission.
Actions by owner
Sender
Recipient
Recipient administrator
- Check the relevant Titan mailbox or domain quota and its reset or account state.
- Restore capacity or communicate the confirmed quota condition before requesting a new send.
Sources
These sources define what this enhanced status code means, mainly through the IANA registry and related RFCs. When provider examples appear on the page, they come from that provider's published documentation. Follow the links to read the original wording in context.
- RFC 3463 — Enhanced Mail System Status Codes — Defines the class/subject/detail model for enhanced status codes.
- RFC 5248 — A Registry for SMTP Enhanced Mail System Status Codes — Creates and governs the IANA enhanced status code registry.
- smtp-codes (community corpus) — Community-maintained reference of provider SMTP responses, pinned locally as evidence.
Last verified:
