What 550 5.2.1 means
5.2.1 is 'Mailbox disabled, not accepting messages'. The address is real, but the account is switched off. RFC 3463 notes it can be permanent or temporary depending on whether the mailbox will be re-enabled; with a 550 basic code the receiver is treating it as permanent.
Gmail documents two different 550 5.2.1 texts. 'The email account that you tried to reach is inactive' is a dead mailbox. 'The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered' is a per-recipient rate limit. Read the text before suppressing.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
550 5.2.1 The email account that you tried to reach is inactive.550 5.2.1 The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered.This variant is a rate limit, not a dead account.
Why cold emailers see 550 5.2.1
- The account was suspended or disabled by the user's admin, often after someone leaves a company but before the mailbox is deleted.
- A consumer account abandoned long enough that the provider marked it inactive.
- Gmail rate-limit variant: the recipient's mailbox is getting flooded.
How to fix 550 5.2.1
- If the text says the account is inactive or disabled, suppress the address.
- If it's Gmail's 'receiving email at a rate' variant, don't suppress; slow down and retry another day.
- Expect more of these from old lists. Disabled accounts usually become 5.1.1 'does not exist' later.
Retry or suppress?
Suppress for 'inactive' or 'disabled'. For the rate variant, wait and retry once or twice before giving up.
Reading the code
The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.2.1, the 5 is the class (permanent failure) and the 2 is the subject (mailbox). The detail number 1 is registered in RFC 3463 / the IANA registry as "Mailbox disabled, not accepting messages".
- Class 5
- Permanent failure
- Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
- Subject 2
- Mailbox
- A problem with the recipient's mailbox, such as full or disabled. Under the recipient's control.
- Detail 5.2.1
- Mailbox disabled, not accepting messages
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.