Can I try sending again later?

No. A 5.1.1 is a permanent failure. Retrying will hurt your deliverability.

Why did an address that worked yesterday bounce today?

The user likely left their job or deleted their account. Corporate email addresses frequently become invalid as employee turnover occurs.

← Error library

550 5.1.1 User unknown / invalid address

permanent Hard Bounce

Meaning

The email address you are trying to reach does not exist on the destination server.

Common causes

The recipient's email address has a typo (e.g., gamil.com instead of gmail.com).
The recipient has left the company, and their email account was deleted.
The user deleted their personal email account.

How to fix

Immediately remove the bounced email address from your mailing list.
Do not attempt to resend to this address, as repeated 550 5.1.1 bounces will severely damage your sender reputation.
Implement double opt-in to catch typos at signup.

Provider notes

Gmail. Returns 'The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces.'

Microsoft 365. Returns 'Bad destination mailbox address' or 'User unknown'.

Amazon SES. Categorizes this as a 'Permanent' hard bounce. Continuing to send to these addresses will increase your bounce rate and risk account suspension.

Example bounce

550 5.1.1 The email account that you tried to reach does not exist.

FAQ

Can I try sending again later?: No. A 5.1.1 is a permanent failure. Retrying will hurt your deliverability.
Why did an address that worked yesterday bounce today?: The user likely left their job or deleted their account. Corporate email addresses frequently become invalid as employee turnover occurs.