If you just want personal email with a minimal amount of CCs involved, or email people within your company who all have the same domain name, this is a fabulous site.
HOWEVER, although they say they can handle thousands of emails per day with a paid service, they actually mean if either of the two conditions I mentioned apply. If you have groups or organizations with completely different email extentions, Fastmail stinks royally.
I did their free trial, during which I spent the better part of the 30 days moving, cleaning up, and organizing my email, calendar, and contacts. I then paid for a one year subscription, and began using the full service, only to have them IMMEDIATELY lock me out of my account with no notice! The reason? "Too many emails", and by that they meant that the 120 or so BCCs I typically send (not thousands, but a bit over one hundred) was more than their infrastructure could handle! They blindly decided it must be spam! In fact, I work with our state political party and state legislature, and the BCCs are to PROTECT personal email addresses. These emails involve county offices, state legislators, and legislative issues!
Even after explaining this the people at Fastmail were adamant I had to move. Of course, no mention of a refund nor any concern about the length of time I'd spent getting everything set up, or the inconvenience and annoyance of having to notify my mailing list of yet another address change in under 45 days! I am still furious by their lack of concern and unprofessional attitude.
In summary, it's an intuitive, easy, robust package but ONLY if you routinely send one-to-one emails and don't carbon many people. If your email needs are more than that, save yourself the headache, and look elsewhere.