A high unknown user rate can affect deliverability as it is a direct reflection of your email address collection and data hygiene practices. A high unknown user rate is often perceived by mailbox providers as a spam characteristic.
Some mailbox providers tolerate unknown user rates up to 10%, but you may see deliverability problems surface when the unknown user rate exceeds 5%.
Senders with strong inbox placement rates usually have unknown user rates less than 2% and the best senders have unknown user rates less than 1%.
