Mailloop sandboxes are not limited to capturing emails your application sends. They can also receive email from any external SMTP sender, making them useful for testing inbound email flows.
How It Works
Every sandbox has an email address in the format [email protected]. Any email sent to this address from any SMTP server on the internet is delivered to the sandbox inbox. No SMTP authentication is required for inbound delivery.
This means you can:
- Send emails from Gmail, Outlook, or any email client to your sandbox
- Configure third-party services (Stripe, SendGrid, notification providers) to send to your sandbox address
- Test how your application handles inbound email by routing replies to a sandbox
Inbox Email Addresses
Each sandbox starts with one email address. You can create additional addresses from the sandbox detail page in the dashboard.
To create an inbox email address:
- Open a sandbox in the dashboard
- Find the Inbox Emails section
- Click Add Email and optionally assign a label
All aliases deliver to the same sandbox inbox.
Use Cases
Testing Third-Party Integrations
Configure a third-party service to send notifications to your sandbox address, then verify the content in the dashboard.
Staging Environment Catch-All
Point your staging environment's reply-to addresses at a sandbox. All customer replies in staging go to Mailloop instead of real inboxes.
End-to-End Inbound Flow Testing
If your application processes inbound email (support tickets, email-to-task, etc.), configure it to poll or receive from a sandbox address during testing. Send test emails from an external SMTP provider and verify your application handles them correctly.
Cross-Service Email Verification
When testing a system where Service A sends email that Service B receives, point Service A's output at a sandbox and verify delivery without involving real mailboxes.
External vs Captured Emails
Emails received from external sources are flagged differently in the dashboard. The quality score analysis is only available for emails captured through SMTP authentication (emails your application sends), since scoring is designed to evaluate your outgoing email quality.
Sending to a Sandbox from the Command Line
You can test inbound delivery using curl or swaks:
# Using swaks (Swiss Army Knife for SMTP)
swaks --to [email protected] \
--from [email protected] \
--server sandbox.mailloop.io \
--port 25 \
--header "Subject: External Test" \
--body "This email was sent from outside."The email appears in the sandbox inbox within seconds.