200

OK

Request processed successfully

401

Unauthorized

Invalid or missing authentication credentials

Verify API key

402

Payment Required

Insufficient account balance

Recharge account

403

Access Forbidden

Access denied for requested resource

Check permissions

404

Resource Not Found

Invalid endpoint or resource ID

Verify request URL

422

Unprocessable Entity

Invalid request parameters

Validate payload fields

500

Internal Server Error

Unexpected server issue

Retry or contact support

502

Bad Gateway

Upstream server failure

Retry after delay

503

Service Unavailable

Server overloaded / maintenance

Retry later

250

Accepted

Email successfully accepted for delivery

421

Service Unavailable

Recipient mail server unavailable

Retry later

452

Insufficient Storage

Mailbox full / storage issue

Retry later

535

Authentication Failed

SMTP credentials invalid

Verify credentials

542

Recipient Invalid

Email address does not exist

Remove recipient

550

Permanent Failure

Email rejected permanently

Investigate rejection reason

4.3.2

System Not Accepting Messages

Recipient system unavailable

Retry later

4.4.5

Mail System Congestion

Server overloaded

Retry later

5.1.1

Invalid Mailbox Address

Recipient mailbox doesn't exist

Remove address

5.7.1

Delivery Refused

Authorization / spam / policy restriction

Review sender setup

1

Queued

Email added to queue

2

Accepted

SMTP accepted email

3

Rejected

Rejected before delivery

4

Delivered

Delivered successfully

5

Opened

Recipient opened email

6

Unsubscribed

Recipient opted out

7

Clicked

Recipient clicked link

8

Bounced

Delivery failed

9

Failed

System-level sending failure

10

Complaints

Marked as spam

deliverable

Valid email address

Safe to send

undeliverable

Invalid email address

Remove immediately

risky

Possible delivery issues

Send cautiously

unknown

Validation inconclusive

Monitor closely

Invalid recipient address

550 / 5.1.1

Mailbox does not exist

Remove recipient

Mailbox full / congestion

452 / 4.4.5

Temporary issue

Retry later

Recipient unsubscribed

550 / 5.7.1 (Event 6)

User opted out

Do not resend

Spam complaint

550 / 5.7.1 (Event 10)

Marked as spam

Remove immediately

Risky validation result

risky

Delivery uncertainty

Monitor sends

Insufficient credits

HTTP 402

Low balance

Recharge account

Invalid request payload

HTTP 422

Incorrect API request

Fix parameters

/api/status-codes

GET

Retrieve all status codes

types

String

Filter code types (2xx,4xx,5xx,failed-logs)

Get HTTP status codes

GET /api/status-codes?types=2xx,4xx,5xx

Get failed logs

GET /api/status-codes?types=failed-logs

Get validation failures

GET /api/status-codes?types=validation-failed-logs

Bounce Rate

Keep below 2%

Permanent Failures

Remove immediately

Unsubscribes

Never resend

Validation

Validate before sending

Webhooks

Enable for real-time updates

Retry Logic

Use exponential backoff

Complaint Monitoring

Keep spam complaints minimal

Documentation

docs.mailer91.com

Support Email

[email protected]