Automatically create SPF records for EC2 instances with static EIP addresses

Currently new EIP machines don't get an SPF record for sending email, so any emails go to spam!

This is challenging for a couple of reasons:

• there may be an existing SPF record, so we would have to find that, read it and preserve existing values, appending our own; and
• SPF records do have a length limit of 255 bytes, we'd have to either warn or fail if it was going to be exceeded and not alter the current record

Obviously this is a solution for small usage, where a handful of machines want to send mail. Anything larger will require a custom setup using either macros or subdomains.