Capably Digital Email Validation and Blocklist

Bounce rates are an indicator of how an SMTP service sees you as a customer, and if your bounce rates get too high it can impact both your inbox placement and whether a provider is willing to continue doing business with you. For the most part, SMTP services don't do their own checks to make sure that you're sending to an email address that makes sense beyond the basics, and will treat an email address where there are no email servers (mx records) as a "hard bounce" that dings you in your providers eyes. Similarly, a blocklist lets you block particular domains and email addresses (for example those that are known to be disposable email addresses) to avoid sending to email addresses that are unlikely to remain monitored.

It depends at which stage the block takes place:

• During email validity checks (hooking into the WordPress is_email filter) the user will be presented with a message advising their email address isn't valid, the same as they would if they used a nonsensical address.
• During email sending (hooking into the WordPress pre_wp_mail filter) the email will not be sent.

Yes, they do. Since email address entries are more specific than domains, those take priority where they exist. This allows you to do things like block an entire domain, but allow a specific email address.

Yes, you can. Simply add the instruction you want the plugin to follow in the Manage Domains page of the plugin's settings, and it will honour that instruction because the implementation for syncing external lists is designed not to override your choices.

No, the implementation for syncing external lists cleans up after itself by removing domains that were added that are no longer part of the list, to keep things manageable.

That depends on your setup. If you're using a mailing plugin that relegates email sending to WP-Cron, then it's unlikely to (but it also depends on where the plugin does this relegation in the process), otherwise, the mx check takes place during the page processing, which can take a moment, but generally not more than sending the email itself.

While we've tried to test as much as possible, it's always possible that a new plugin has bugs (whether that's a missed error or something specific to your environment). If you have an issue we recommend enabling logging, which will generally tell you why the plugin blocked an email. If you still have issues, head to the plugin support forum and leave a message, and we'll do our best to work with you to solve any issues you have. Don't forget, that if you're having issues, others may too, so let us know!