7 Reasons Why Your Emails Are Not Arriving (and How to Fix Each One)

You sent an email. The recipient says they never got it. No bounce, no error, just silence. This is one of the most frustrating problems in email — and it is almost always caused by one of these seven issues.

1. Missing or broken SPF, DKIM, or DMARC

This is the #1 cause of email delivery failures in 2026. Since Google and Yahoo enforced authentication requirements in February 2024, emails without proper SPF, DKIM, and DMARC are increasingly rejected or sent to spam.

How to check:

nslookup -type=TXT yourdomain.com        # Check SPF
nslookup -type=TXT _dmarc.yourdomain.com  # Check DMARC

How to fix: Follow our complete guide to email authentication. All three records (SPF, DKIM, DMARC) should be configured and passing.

2. Your IP address is on a blacklist

If your mail server's IP (or your email provider's shared IP) is listed on a DNSBL blacklist, receiving servers may silently reject your email.

How to check: Search your sending IP on MXToolbox Blacklist Check.

How to fix: Follow the delisting procedure for each blacklist provider (Spamhaus, Barracuda, SpamCop each have their own process). Using Cleanbox Relay in front of your server means Cleanbox's IPs are what recipients see, protecting your server from direct blacklisting.

3. The recipient's spam filter caught it

Your email may have been delivered — to the spam folder. Common triggers:

• Subject lines with ALL CAPS, excessive exclamation marks, or spam phrases
• Image-heavy emails with little text
• URL shorteners (bit.ly, tinyurl) in the body
• Sending from a new domain with no reputation

How to fix: Ask the recipient to check their spam/junk folder and mark your email as "not spam." See The Complete Guide to Email Deliverability for Small Senders for content best practices.

4. DNS records are misconfigured

If your MX records point to the wrong server, or your domain's DNS is not resolving properly, email to your domain will fail silently.

How to check:

dig MX yourdomain.com +short

How to fix: Verify your MX records point to the correct mail server. If you use Cleanbox Relay, MX should point to mx1.cleanbox.to and mx2.cleanbox.to. See DNS configuration guide.

5. The recipient's mailbox is full or inactive

A full mailbox returns a temporary error (4xx), which causes the sending server to retry. If the mailbox stays full beyond the retry window, the message eventually bounces. An inactive or deleted mailbox returns a permanent error (5xx) immediately.

How to fix: This is on the recipient's side. If you see a bounce message mentioning "mailbox full" or "user not found," there is nothing you can do except contact them through another channel.

6. Email size exceeds limits

Most mail servers reject emails above 25 MB. Some are stricter (10 MB). Large attachments are the usual cause.

How to fix: Use a file sharing service (Google Drive, Dropbox, WeTransfer) for large files instead of attaching them directly. Cleanbox's SMTP limit is 25 MB per message.

7. Your sending server has no reverse DNS (rDNS)

Receiving servers check if the sending IP has a valid PTR record (reverse DNS). Missing rDNS is a red flag — it suggests the server is a home computer, compromised machine, or hastily set up infrastructure.

How to check:

dig -x YOUR_IP_ADDRESS +short

How to fix: Configure a PTR record for your mail server's IP through your hosting provider. The PTR record should resolve to a hostname that matches your server.

Quick diagnostic checklist

Symptom	Most likely cause	Check
No bounce, email just gone	Spam folder or silent rejection	Ask recipient to check spam; check blacklists
Bounce: "550 Authentication failed"	SPF/DKIM/DMARC failure	Check DNS records
Bounce: "550 User not found"	Recipient address does not exist	Verify the address
Bounce: "452 Mailbox full"	Recipient's mailbox is full	Contact them another way
Bounce: "552 Message size exceeds limit"	Email too large	Reduce attachments
Emails delayed by hours	Greylisting or server queue	Wait; usually resolves within 30 minutes