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550 5.2.1 error: the mailbox is inactive

550 5.2.1 means the mailbox exists but isn't accepting mail; Gmail uses it for inactive accounts. For outreach, treat it as a dead address.

List problemThe address is the problem. Clean the list; your domain is fine.

What 550 5.2.1 means

Any server5.2.1 Mailbox disabled, not accepting messages

The standard meaning: the mailbox exists but isn't taking mail.

Gmail550 5.2.1 The email account that you tried to reach is inactive.

The Gmail account is inactive.

Gmail550 5.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered.

A different case: that one recipient is getting too much mail right now. Try again later.

List problem or sender problem?

List problem. The account is disabled or inactive, which your setup didn't cause. For outreach it behaves like an address that doesn't exist. It's a permanent failure: the server won't accept this message if you send it again unchanged.

How to fix 550 5.2.1

  1. Remove the address from your sequences and reach the person another way.
  2. For the rate version, wait and try once more later.

In SendingOS, a 550 5.2.1 bounce shows its reason and code on the conversation, and the thread is labelled Bounced, so you can filter the inbox to bounces and pull these addresses before they add up.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

550 5.1.1552 5.2.2450 4.2.1

Sources: Google: Gmail SMTP errors and codes · RFC 3463