What are the most common causes of soft bounces?

You send an email, and instead of landing in the inbox, it bounces back with some cryptic message about a busy server or a full mailbox. That's a soft bounce. Unlike a hard bounce, which is permanent, a soft bounce is temporary. The receiving server is essentially saying "not right now" instead of "not ever." Your ESP will usually retry automatically for a day or two before giving up.

Here's what's actually causing them, and how to tell the difference.

Mailbox-side issues are the most common. The subscriber's inbox is full (over their storage quota), their mailbox is temporarily disabled, or the account is suspended. These show up in your bounce logs as 452 or 552 codes. If you're seeing a mailbox full error, it often clears itself in a day or two. But if it keeps bouncing the same address over multiple sends, that's a sign the mailbox has been abandoned.

Receiving server issues come next. The mail server at the destination is overloaded, down for maintenance, or experiencing an outage. These usually resolve on their own and your ESP's retry logic handles them quietly. You'll see 421 codes here.

Rate limiting from the receiving end means the destination server is throttling you specifically. You're sending too fast, or too many connections are hitting their server at once. Gmail and Outlook do this regularly when they don't recognize your sender reputation yet. Slow down your sending pace and this usually eases up.

Greylisting is an intentional delay by spam filters for senders they haven't seen before. The server rejects your email temporarily, then accepts it once you retry. Most ESPs handle this automatically. If you're curious about how it works, there's a full explanation in the greylisting question.

DNS and connection timeouts happen when the lookup for the receiving server stalls or a network issue breaks the connection mid-attempt. These are usually infrastructure hiccups that resolve within hours.

Content-based soft bounces are trickier. Some servers will temporarily reject a message if it looks suspicious or triggers a policy check without being outright spam. If the same message is bouncing across multiple addresses, check your content for spam signals before assuming it's a server problem.

The pattern matters more than any single bounce. One soft bounce on one address is background noise. The same soft bounce type repeating across many addresses, or the same address soft-bouncing on three or more consecutive sends, tells you something real is wrong. Most ESPs will automatically convert a persistent soft bounce to a hard one after a few failed retries (the exact threshold varies by platform).

But if your soft bounce rate suddenly spikes, that's worth investigating. A jump in rate-limiting bounces often points to a sending reputation problem. A jump in mailbox-full bounces can signal a stale list. You can dig into your bounce codes directly inside your ESP's reporting, or run your domain through our free blocklist checker to rule out a reputation issue on your end.