What 550 Mailbox unavailable Means
550 is the most common permanent failure in SMTP. Per RFC 5321, it means the requested action wasn't taken — the mailbox was unavailable for some permanent reason. The exact cause depends on the text that follows (e.g. 'no such user', 'unable to relay', 'message rejected') or the enhanced status code (5.1.1, 5.7.1, 5.2.2, etc.).
Every SMTP receiver returns 550 for various permanent failures. Major receivers usually pair it with an enhanced status code (5.x.x); smaller or older receivers may return bare 550 with descriptive text.
Recipient doesn't exist (most common); recipient quota exceeded permanently; receiver policy rejection (sender on blocklist, message content flagged); relay denied (you're sending to a domain that the receiver doesn't host); or the receiver doesn't accept your IP for that domain.
How to Fix 550 Mailbox unavailable
- 1
Read the text and enhanced status code
Bare 550 is generic. The descriptive text or enhanced code tells you the real cause. '550 5.1.1' = user unknown. '550 5.7.1' = policy block. '550 5.2.2' = over quota. '550 5.7.26' = unauthenticated email. Each has a different fix.
- 2
If user unknown — verify and remove
If 550 indicates the recipient doesn't exist, the address is invalid. Run your list through a verification service before resending. Most cold email lists have 5-15% invalid addresses; a verification pass eliminates them up front.
- 3
If policy block — check IP and domain reputation
Policy-block 550s mean the receiver rejected your IP or sender domain. Check the IP at coldrelay.com/tools/blacklist-checker. Check your domain's reputation via Google Postmaster Tools. Run the Email Deliverability Test to confirm SPF, DKIM, DMARC are all passing.
- 4
If quota — let it be
If 550 indicates the recipient is over quota, you can't fix it. Some verification services flag known-perpetually-over-quota addresses as 'role accounts' so you can exclude them up front.
- 5
If unable to relay — wrong destination
If you're seeing '550 unable to relay' from a server, that server doesn't host the destination domain. Either your MX record lookup is wrong, or you're routing through a server that doesn't accept your destination. Use the MX Lookup tool at coldrelay.com/tools/mx-lookup to confirm the correct MX for the recipient domain.
References
- ◇
- ◇RFC 3463 — Enhanced Mail System Status Codes
Defines the 5.X.X enhanced status codes that narrow 550's cause.
550 Mailbox unavailable in the Cold Email Context
550 is the cold email sender's most common headache because it covers everything from list quality to reputation problems. The fix depends entirely on the enhanced status code or text. List-quality 550s (user unknown, no such mailbox) are addressed by pre-send verification. Reputation 550s (policy block, content rejection) need infrastructure remediation — clean dedicated IPs, correct authentication, proper warmup. ColdRelay's approach: dedicated infrastructure prevents the reputation-class 550s, and the bounce-classification pipeline automatically tags user-unknown 550s so they're excluded from future campaigns without manual list hygiene.
Frequently Asked Questions
Is 550 the same as a hard bounce?
Most often yes. 550 is permanent (5xx), so the sender should give up. 'Hard bounce' is the colloquial sending-platform term for the same thing. Some sending platforms distinguish between user-unknown 550s (true hard bounce) and policy-block 550s (still hard bounce but for a different reason).
Can I appeal a 550?
Only if the cause is a policy block from a major provider. Microsoft 365 has a delisting form. Gmail allows reputation appeals via Postmaster Tools. User-unknown and quota 550s can't be appealed — they're definitional.
Does 550 hurt my reputation?
Yes, indirectly. A high 550 rate signals you're sending to bad addresses or to recipients who don't want your mail. Receivers track 550 rates and adjust your IP's reputation accordingly. Keep your 550 rate under 2% to maintain good standing.
Should I retry 550?
No. 550 is permanent. Retrying produces the same result. Most sending platforms treat 550 as a terminal bounce and mark the address invalid.