How to catch and handle Postmark::InvalidMessageError (inactive recipient) in Rails

# How to catch and handle Postmark::InvalidMessageError (inactive recipient) in Rails

## The problem

Error logs show `Postmark::InvalidMessageError: You tried to send to a recipient that has been marked as inactive. Found inactive addresses: ...` Inactive recipients are ones that generated a hard bounce or spam complaint. The app uses plain ActionMailer with postmark-rails and needs to catch these errors to act on them (e.g. mark the address as bounced) instead of crashing.

## The verified fix

Rescue the specific Postmark exception class around the deliver call:

```ruby
begin
  postmark_client.deliver_with_template(...)
rescue Postmark::InvalidMessageError => error
  # mark as bounced or take other action
end
```
If you need finer control, inspect `error.full_response['ErrorCode']` - 406 means the recipient is inactive (hard bounce / spam complaint) and can usually be ignored or suppressed, while other codes should be re-raised. Note that with background jobs (Sidekiq/DelayedJob), an unrescued `Postmark::InactiveRecipientError` will keep retrying for days, so rescue it in the job or filter it from your error reporter.
Source: https://github.com/ActiveCampaign/postmark-rails/issues/19