Handle 504 Gateway Timeout when sending messages to Mandrill

Frequently we're seeing this error when using Mandrill (Mailchimp Transactional API):

```html
<html><body><h1>504 Gateway Time-out</h1>\nThe server didn't respond in time.\n</body></html>
```

Figure out the best way to handle this error, and what it implies - does it mean that the message was sent?