What are “alias rejections”?
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You send an email to info@somecompany.com, it bounces, and the error says "user unknown." But you've emailed that address before and it worked fine. What happened? You probably hit an alias rejection.
An email alias is a shortcut address that forwards to a real mailbox. Think info@, support@, hello@, or old addresses a person used before changing their name. The alias itself doesn't store email. It just points somewhere else.
An alias rejection is what happens when that shortcut breaks. The address exists (or used to), but the mail server can't complete the delivery because the forwarding chain fell apart.
There are a few ways this happens. The alias gets deleted when someone leaves a company and IT removes the old email address. The target mailbox gets deleted but the alias is left dangling, still pointing at nothing. The alias gets temporarily disabled. Or someone misconfigured the forwarding setup and it just never worked the way it was supposed to.
The frustrating part is that the bounce message often doesn't tell you an alias was involved. You'll see a generic "user unknown" or "address does not exist" error, same as any hard bounce. There's no asterisk saying "this used to forward somewhere."
So how do you handle them? If the same address bounces consistently across two or three sends, treat it as a hard bounce and suppress it. The alias is gone and it's not coming back on its own. If it's intermittent (bounces once, then delivers fine), it may have been a temporary configuration issue worth a single retry. But don't keep hammering it. Repeated sends to a dead address hurt your sender reputation just as much as any other bad address.
The honest reality is that alias rejections are hard to distinguish from regular invalid addresses. Your ESP won't label them differently. The safest approach is to treat any consistent bounce as a signal to suppress, alias or not. If you're seeing a spike of "user unknown" bounces from a single domain, it's worth checking whether that company recently restructured (and may have retired a batch of old forwarding addresses in one go).
If your bounce rate is climbing and you're not sure what's driving it, a list clean can help surface patterns across your suppression data. We do that if you want a second set of eyes on your list.
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