What does “invalid recipient address” mean?

You see "invalid recipient address" and your first thought is probably: "But I checked that list." The issue is that somewhere in the validation chain, something about the address itself isn't meeting the server's requirements.

This error usually means one of a few things. The address format is broken (missing the @ symbol, invalid characters, a domain that doesn't have proper structure). The mailbox doesn't actually exist. The server's validation rules rejected it based on local policies. Or the address syntax is just malformed on a technical level.

You'll see this paired with bounce codes like 550 (permanent failure) with "invalid recipient," or 553 saying the mailbox name isn't valid. These are hard bounces. which means the address won't accept mail at all.

Here's the practical move: Check a sample of those addresses manually. Look for typos in the domain, missing TLDs, extra spaces, or weird characters. If they genuinely look fine to you, the validation might be provider-specific (some servers are picky). Remove the confirmed invalid addresses from your list. If it's a data entry problem at the source, fix that in your form or import process. Most ESP platforms let you suppress addresses automatically after they bounce, which prevents repeat problems. (Of course, easier said than done if your list is huge.)

The next step: Run a quick validation check on your list before the next send. Our bounce analysis tools can help you spot patterns in why addresses are failing.