5.1.1 means the mailbox does not exist at the receiving domain. It is permanent. Suppress the address immediately. Continuing to send to addresses that return 5.1.1 is one of the fastest ways to damage sender reputation.
What it looks like in the log
550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table
550 5.1.1 The email account that you tried to reach does not exist.
550 5.1.1 unknown or illegal alias
Why it happens
The address was mistyped at collection.
The mailbox was deleted, commonly after someone leaves an organisation.
The address was never real, which points at list quality or a lack of confirmed opt-in.
What to do
Suppress permanently on the first occurrence. There is no reason to retry.
If the hard bounce rate is above about 2%, the problem is list acquisition, not delivery.
Validate at the point of capture rather than discovering invalid addresses at send time.
This response maps to invalid_recipient with a recommended action of
suppress in
smtpsift, an open-source classifier for
SMTP rejections and deferrals.