550 5.2.1: Mailbox disabled or not accepting mail

550 5.2.1 means the mailbox exists but is not accepting messages right now. Gmail also uses the same code for a recipient who receives mail at a rate that blocks more. Read the text to see which one you have.

Bounce type

Hard bounce (permanent)

Usually caused on

The recipient's side

Terlido report label

Invalid recipient

What the standard says

RFC 5321 defines 550 as "mailbox unavailable". RFC 3463 defines 5.2.1 as "Mailbox disabled, not accepting messages": the mailbox exists, but it is not taking mail. The RFC notes this can be permanent if the mailbox will never be turned back on. The 5 in front says the receiving server considers it permanent.

What the mailbox providers say

• Gmail (550 5.2.1): “The email account that you tried to reach is inactive.” Source
• Gmail (550 5.2.1): “The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered.” Source

Common causes

• The account was suspended or disabled by its administrator, for example after a person left.
• The account is inactive and the provider turned it off.
• On Gmail, the recipient is receiving a very high volume of mail, and Gmail stops adding more for now.
• A shared mailbox that was closed but whose address still sits on your list.
• An employee on leave whose account the administrator switched off for the time being.
• A former customer or user account on a free provider that closed the account after a long period without use.

How to fix it

• If the text says the account is inactive, remove the address. It is unlikely to come back.
• If the text says the recipient receives mail at a high rate, the mailbox may take mail again later. Any 5.x.x code is a hard bounce in Terlido, so the address is blocklisted first. Remove the blocklist entry only if you want to try that one contact again after some days.
• Check whether you contacted the same person at several addresses in one company, because a burst to one inbox can trigger the rate wording.
• Keep the bounce text, not only the code. Both meanings share 5.2.1, so the text is the only way to tell them apart later.
• Do not retry an inactive account several times. Each retry that bounces counts against you.
• When you remove contacts for this reason, keep a note so the same address is not imported again from another source.

Is it a deliverability problem?

The inactive-account meaning is a list quality issue, like 5.1.1. The rate-of-receipt meaning is not a fault of your domain. The cause is the total volume of mail reaching that inbox, which you cannot change. The part you can change is your own share of it: space out contacts at the same company and keep the number of touches per person low. Fewer touches from fewer senders at the same company lower the chance of this reply.

How Terlido handles it

When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 5 is a hard bounce. In the campaign Reports tab this one is grouped as "Invalid recipient".

On a hard bounce Terlido adds the address to the workspace blocklist with the reason "hard bounce", so no campaign emails it again. The lead stops in that campaign as bounced, its lead score goes down by 15, and the bounce counts toward the campaign's bounce rate.

Once a campaign has at least 20 sends, it pauses itself when hard bounces divided by sends goes over the campaign's maximum bounce rate. The default is 5%. If the cause was on your side and you fixed it, you can remove the entry on the Blocklist page of your account settings and try again.

Free tools that help

• Email list format checkerReview CSV structure, empty addresses, and basic email syntax without contacting mail servers.
• Bounce rate calculatorCalculate bounced messages as a share of attempted sends.

Related error codes

• 550 5.1.1: Recipient address does not exist
• 552 5.2.2: Mailbox full
• 452 4.2.2: Mailbox full (temporary)

Sources

• RFC 5321: Simple Mail Transfer Protocol (reply codes, section 4.2)
• RFC 3463: Enhanced Mail System Status Codes
• Google: Gmail SMTP errors and codes