---
title: "Advanced knowledge"
description: "Explore how BooleanSMTP chooses a mailer, handles multiple senders and failed sends, and supports safe testing and credential storage."
image: "https://booleansmtp.com/docs/og.png"
---

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

# Advanced knowledge

Start with the question you need to answer:

- [How is a connection chosen?](/docs/advanced/how-a-connection-is-chosen/) Follow the order used for an outgoing message.
- [How can I use multiple mailers?](/docs/advanced/multiple-mailers/) Set up distinct From addresses and compare common routing scenarios in BooleanSMTP Free.
- [Why can only one connection use a From address?](/docs/advanced/one-connection-per-sender/) Resolve a sender conflict while saving or activating a connection.
- [What happens when a send fails?](/docs/advanced/fallback-and-retries/) Understand immediate fallback and later retries.
- [Can I test without sending real email?](/docs/advanced/simulation-mode/) Capture messages in Email Logs with simulation mode.
- [Where can I keep provider credentials?](/docs/advanced/wp-config-credentials/) Use supported configuration storage instead of the database.

Source: https://booleansmtp.com/docs/advanced/index.mdx