Adding "contactable via email" will not affect the sending of marketing emails as the same rule gets applied automatically when sending those regardless of what target expression you enter. It will exclude global unsubscribes and people with invalid emails from service emails, though, so would definitely affect bounce rates there.

How Taguchi classifies the various error types

Taguchi uses quite a number of rules to first classify bounce messages by error type (for example, is the receiving mail server saying the mailbox does not exist? or is it rejecting the message due to suspected spam content? or is it an out-of-office autoreply?).

Following that, we classify the various error types into High Level Categorisation:

• Network failures Where server is uncontactable.
• Permanent errors Mailbox does not exist - and thus email cannot be delivered. Other causes can be found here.
• Temporary errors Receiving Mailbox has ran out of storage space (e.g. "mailbox is full").
• Non-errors (e.g. out of office messages).

This classification is then overlaid on a set of per-inbox-provider rules, because some providers incorrectly return permanent errors for issues that are actually temporary (or vice versa).

Finally, we look at the bounce and activity pattern for an individual subscriber before determining that their email address should be flagged as invalid.

This is a holistic processing looking at the number of bounces of different types:

• Stability of the bounce type (e.g. if we get a permanent error and then a temporary error, that's less likely to be permanent than two permanent errors)
• Engagement history (e.g. if we get a permanent error, a successful send with open/click, and then a permanent error, that address is unlikely to be invalid).

Learn more about What is an email bounce? and Why does an email bounce?.

This process takes up to a week in the normal case (since bounces can come in several days after the message was initially delivered), and as inbox providers update their rules from time to time we update the rules at our end and sometimes also re-process bounce activity or invalid flags.