A soft bounce means the receiving mail server temporarily could not accept your message. The straight answer to what causes email soft bounces is usually a full mailbox, a temporary server problem, a message that is too large, or a receiving system that does not trust the message enough to accept it right now.
That word, temporarily, matters. A soft bounce is not the same as proof that an address is bad. But if the same address keeps soft bouncing, or many messages begin soft bouncing at once, it can point to a delivery or reputation problem that needs attention.
For a business sending invoices, appointment reminders, listing updates, or client documents, a soft bounce can be easy to overlook. The message may be retried automatically, but the recipient may still miss a time-sensitive email. Knowing the reason behind the bounce tells you whether to wait, contact the recipient another way, or fix a problem with your own sending setup.
What causes email soft bounces?
Email servers return a response when they accept or reject a message. A soft bounce generally comes with a temporary response, often in the 400 status-code range. Your email platform may label it as deferred, delayed, pending, or temporarily rejected instead of using the phrase “soft bounce.”
The exact wording from the recipient’s server is the best clue. “Mailbox full” has a different fix from “message rate limited” or “authentication failed.” Do not treat every soft bounce as a list-cleaning issue. First, separate a recipient-side delay from a sender-side issue.
1. The recipient’s mailbox is full
This is the most familiar cause. The recipient has used the storage allocated to their mailbox, so their provider cannot accept another message. The address may still be valid, and your message may go through after the recipient clears space.
There is little you can fix on your end. Your sending system should retry the message for a reasonable period. If the message is urgent, use a known alternate contact method rather than repeatedly resending it.
2. The recipient’s mail server is temporarily unavailable
A receiving server can be offline for maintenance, overloaded, misconfigured, or briefly unable to communicate with other servers. Internet routing problems can produce the same result.
One or two messages delayed this way are normal. A pattern affecting many contacts at the same company may mean their email system has a problem. A pattern across many unrelated recipients is more likely connected to your own sending service, message volume, or domain reputation.
3. Your message or attachment is too large
Every email provider sets size limits. Those limits include the message body, headers, embedded images, and attachments. An attachment can also grow when email encoding is applied, so a file that appears to be under a limit on your computer can exceed the recipient’s accepted message size.
The exact fix is simple: send a smaller file, reduce image dimensions, or use your approved document-sharing process when appropriate. Do not assume a large PDF or photo-heavy proposal will be accepted just because it sent successfully to other people. Recipient limits vary.
4. The receiving server is slowing your mail down
Mail providers use rate limits to manage traffic and reduce abuse. If you send a high volume of messages to one provider or company in a short period, that provider may temporarily defer additional mail. This does not automatically mean you are sending unwanted email. It can happen after a legitimate campaign, a large customer notification, or a busy day of transactional messages.
The right response depends on the pattern. Slow the sending rate, spread non-urgent messages over time, and confirm that your mail system is handling retries rather than creating duplicate sends. If your business sends both marketing and operational email, keep those streams organized so a high-volume send does not interfere with important client communication.
5. Your domain authentication is missing or misaligned
Some servers temporarily reject mail when they cannot verify who is allowed to send for your domain. This is where technical records matter.
SPF, or Sender Policy Framework, is a Domain Name System (DNS) record that identifies services authorized to send mail for your domain. DKIM, or DomainKeys Identified Mail, adds a cryptographic signature that receiving servers can validate. DMARC, or Domain-based Message Authentication, Reporting, and Conformance, tells receiving servers how to evaluate SPF and DKIM results and whether those results align with the visible From address.
Authentication failures can produce a soft bounce, a hard rejection, spam filtering, or inconsistent results depending on the recipient’s policy. The trade-off is that a record can exist and still be wrong. For example, an old newsletter platform, office copier, customer relationship management system, or invoice tool may be sending mail without being included in SPF or signing with DKIM.
Review every system that sends as your domain. Then verify that its authentication is configured correctly. Avoid making random DNS edits to “fix” a single error message. Incorrect SPF changes can prevent other legitimate services from sending.
6. Your sending reputation has weakened
Receiving providers assess signals associated with the sending domain and server. Those signals can include past complaint activity, repeated sends to inactive or invalid addresses, sudden volume changes, and whether the server has appeared on a blocklist.
A blocklist is a list of domains or internet addresses identified as possible sources of unwanted or harmful mail. Being listed does not always cause a soft bounce, and a soft bounce does not prove a listing. Still, reputation trouble can lead a provider to defer mail while it applies additional checks or limits traffic.
Start with the bounce details and look for a pattern. Are the delays tied to one provider? Did they begin after a new sending tool was added? Did sending volume rise quickly? The exact fix may involve correcting authentication, removing invalid contacts through normal list maintenance, or adjusting how your systems send. Do not keep resending to addresses that repeatedly fail without investigating why.
7. The recipient’s security rules need extra verification
Some organizations use strict inbound mail controls. Their servers may temporarily defer messages if they cannot complete a security check or if your server’s configuration looks unusual. Reverse DNS, also called PTR or pointer records, is one example. It connects the internet address of a sending server back to a recognizable hostname.
Transport Layer Security (TLS) protects email traffic while it moves between servers. MTA-STS, or Mail Transfer Agent Strict Transport Security, lets a domain publish rules for requiring secure TLS connections. TLS-RPT, or TLS Reporting, provides reports about problems with those encrypted connections. These standards are not the cause of every soft bounce, but connection and identity issues can affect whether a receiving server accepts your mail.
If you use a major email service, some server settings are managed for you. If you operate your own mail server or use a specialized sending application, the investigation may require access to DNS, server logs, and SMTP, or Simple Mail Transfer Protocol, connection settings.
How to tell whether a soft bounce is harmless or serious
A single soft bounce is usually a wait-and-watch event. Your sending platform should retry it automatically, often more than once. Repeated manual resends can make the situation worse by increasing volume and creating duplicate messages.
A recurring pattern deserves action. Look at how many messages are affected, which recipients are involved, and the exact response returned by the receiving server. If one client’s mailbox is full, the issue is local. If messages to multiple providers are delayed, investigate your domain and sending configuration.
Also check whether the bounce reason changes. A mailbox-full notice may later become a successful delivery. An authentication error will usually persist until the sending system or DNS record is corrected. Keep a record of the original bounce message, date, sender address, recipient domain, and sending platform. That information gives your IT person or email provider something concrete to diagnose.
A practical response to soft bounces
Start by reading the bounce notice, not just the label in your email dashboard. Identify whether the response points to the recipient, the content, the message size, traffic limits, or trust in your sender identity.
Next, let legitimate temporary failures retry on their normal schedule. For urgent one-to-one communication, confirm the recipient’s alternate address or phone number through an established contact method. For recurring or broad delays, pause and inspect the systems that send mail as your domain.
Finally, verify the basics that affect trust: SPF, DKIM, DMARC, MX records that direct incoming mail, reverse DNS for self-managed servers, and any blocklist signals. These checks will not explain every full mailbox, but they can identify sender-side problems before they turn into repeated delays or rejected mail.
If soft bounces are recurring or affecting multiple recipients, run MailArrive’s free email health check and review the Report Card. It grades your domain and shows the plain-language findings you need to identify the next exact fix.