Re: Mail Migration
Get in touch with Audriga (https://www.migrate-mail.com/en.html). They don't just do "cloudy" migrations (i.e. via their own servers), they can also supply a sort of black box approach. This is, of course, assuming you do a transfer between two DIFFERNT systems (for instance, from Exchange to an Open Source solution), so what you'll do is log into an account and pump emails into the target.
Your main challenges are (1) legality (privacy needs to be protected so you're going to have to get the process inspected or get people to sign agreements), (2) bandwidth, and (3) getting everyone's passwords, although you can just give an outage time and do a password reset instead.
Good luck - it doesn't actually take as long as you think, but planning, backups and test runs are essential. However, one warning: the use of Google may put you on the wrong side of Data Protection laws. Regulators seem to be reluctant to mention this publicly (I assume it's politics getting in the way), but as a sender never has the ability to approve you passing on an email to a 3rd party you are apparently breaking Data Protection every time you receive an email. Just give the ICO a call - they will confirm this (at least, they did when I called). You may want to keep this in mind - don't let the illusion of "free" cloud your judgement (sorry about the unintentional pun).
Biting the hand that feeds IT
