Email Troubleshooting¶
Start here when mail is bouncing, sending fails, messages are missing, or a mail app will not connect. Use the broad error guide first, then move to bouncebacks, headers, or settings as needed.
Quick Reference¶
Sending Problems¶
| Symptom or error | Usually means | Start here |
|---|---|---|
| Cannot send from a mail app | SMTP authentication, password, port, or outgoing server setting is wrong. | Find email settings. |
SMTP authentication error |
The app is not authenticating with the full email address and mailbox password. | Check sending settings. |
Port 25 does not work |
The internet provider or network may block outbound port 25. |
Use port 587 with TLS in the email settings finder. |
| Messages bounce back | Recipient, domain, quota, SPF, reputation, or spam filtering issue. | Review bouncebacks. |
554 Denied or spam rejection |
The receiving server rejected the message. | Save the bounce and headers. |
550 5.7.1 [...] from MailChannels |
Fused outbound filtering rejected or paused the message. | Review MailChannels errors. |
| SPF error | The sending server is not authorized for the domain. | Review SPF. |
Receiving Problems¶
| Symptom or error | Usually means | Start here |
|---|---|---|
| Cannot receive mail in an app | Incoming server, password, SSL, port, or local app cache issue. | Find email settings. |
| Webmail works but the app does not | The mailbox is working; the issue is likely device or client configuration. | Check settings. |
| Webmail login fails | The mailbox password may be wrong or the mailbox may not exist. | Reset the email password. |
Mailbox full or quota reached |
The mailbox or account is out of space. | Manage mailbox quota. |
| Old messages download again | Mailbox cache or quota issue. | Review common email problems. |
| Messages are missing | Filters, forwarding, spam folders, or local POP settings may be moving mail. | Review filters. |
Bouncebacks And Delivery¶
| Symptom or error | Usually means | Start here |
|---|---|---|
550 No Such User Here |
The recipient address does not exist on the destination server. | Check the address. |
No Such Domain |
The recipient domain is misspelled or does not resolve. | Review bouncebacks. |
550 Sender IP reverse lookup rejected |
The recipient server is rejecting based on sender server checks. | Send Fused the bounce. |
| Reputation or blacklist message | A recipient-side filter, blocklist, or reputation system rejected mail. | Save headers. |
| Support asks for headers | Headers show the path and authentication checks for a message. | View full headers. |
Security, Spam, And Older Clients¶
| Symptom or error | Usually means | Start here |
|---|---|---|
| Too much spam reaches the inbox | Additional mailbox or account filters may be needed. | Review spam options. |
| Legitimate mail is landing in spam | Spam scoring, filters, forwarding, or external provider filtering may be involved. | Review filters. |
| BoxTrapper challenge messages appear | Challenge-response filtering is active for the mailbox. | Review BoxTrapper. |
| Certificate warning in Outlook 2007 or older software | The device may not trust the Let's Encrypt root certificate. | Add Let's Encrypt certificates. |
| Password may be compromised | The mailbox password should be reset and webmail security reviewed. | Reset password. |
MailChannels Outbound Filtering Errors¶
Fused uses MailChannels for outbound email filtering. This helps protect shared mail reputation by scanning outgoing messages for spam, compromised accounts, bad recipient lists, suspicious forwarding, and reputation problems before mail leaves the network.
When MailChannels blocks a message, the bounce usually contains a code such as
550 5.7.1 [CS] Message blocked. Save the full bounce message before changing
settings. Fused support can use the sender, recipient, subject, timestamp, and
MailChannels code to review what happened.
| MailChannels code | Usually means | First response |
|---|---|---|
[CS] Message blocked |
The message content was classified as spam or highly suspicious. | Save the bounce and message details; contact Fused if the message is legitimate. |
[SS] |
The subject line matched spam-like patterns. | Rewrite the subject, remove spammy wording, and confirm the mailbox is not compromised. |
[FBL] |
A recipient complained about mail from the sender or account. | Review consent, mailing list hygiene, and unsubscribe behavior. |
[IRR] |
The sender is hitting too many invalid recipients. | Clean the recipient list and remove invalid addresses before sending again. |
[ESA] sender blocked |
MailChannels detected repeated or confirmed abuse from the sender. | Secure the mailbox or script, then contact Fused for review. |
[RBL] Sender blocked |
The sending IP, domain, or related reputation source is listed on a block list. | Save the bounce; Fused can review whether the listing is sender-specific or network-side. |
[FWSS] |
Forwarded spam or compromised forwarding behavior may be involved. | Review forwarders, filters, and the original source of forwarded messages. |
[NS] |
Messages with no subject or suspicious no-subject traffic were detected. | Add clear subjects and verify the account is not sending automated junk. |
[SHF] Message rejected |
The message appears to impersonate another sender or brand. | Confirm the sender identity, domain alignment, and message purpose. |
What To Send Fused¶
Include:
- The full bounce message.
- Sender address.
- Recipient address.
- Subject line.
- Approximate time sent, including timezone.
- Whether the message came from a mail app, webmail, website form, plugin, script, or bulk mailing tool.
- Whether other messages from the same account are sending successfully.
What To Check First¶
Before asking Fused to review a block, check the likely source:
- Mailbox password: reset it if compromise is possible.
- Devices: scan computers that use the mailbox.
- Website forms and plugins: check for spam submissions or outdated code.
- Forwarders: confirm they are not forwarding inbound spam outward.
- Mailing lists: remove invalid recipients, old contacts, and anyone who did not opt in.
- Message content: remove misleading subjects, URL shorteners, suspicious attachments, and sales language that looks like spam.
Do not keep retrying blocked mail
Repeated retries can make a reputation or sender block worse. Save the bounce, correct the likely cause, and contact Fused when the message appears legitimate.
Email Checks¶
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Email errors
Start here for sending, receiving, bounceback, and mailbox errors.
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Bouncebacks
Decode common bounce messages and delivery failures.
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Common email problems
Review quota issues, spam filtering, and outgoing SMTP authentication.
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Full message headers
Find the full headers support may need to trace an email issue.
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Email settings
Confirm IMAP, SMTP, webmail, and device settings for a mailbox.
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Older mail clients
Fix Let's Encrypt certificate trust errors in legacy clients like Outlook 2007.