> ## Documentation Index
> Fetch the complete documentation index at: https://www.plain.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Email deliverability troubleshooting

> How Plain handles email delivery, retry logic, bounce types, SMTP error codes, and common troubleshooting gotchas.

If you are experiencing errors when sending an email, we first recommend checking out our set up guide for sending emails with Plain.

## Best practices

### Warm-up your domain

A common cause of email deliverability issues is trying to send too much, too soon. When you have only recently onboarded a new vendor like Plain to send emails, we recommend slowly ramping up your activity.

### Use a sub-domain for non-Support email sends

Use a subdomain (like `marketing.atrium.co`) for non-Support customer communication.

## Known errors

### 550 DMARC check failed

Occurs if your SPF record has the `-all` strict flag.

### 550 5.7.515

This error occurs if you send more than 5000 emails to Microsoft Consumer Inboxes using the same 5322.From address.

### 550 5.7.26

Returned when there is a misconfiguration in your SPF, DKIM or DMARC records.