I switched to Postbox from Thunderbird. I paid US$9.95 after the 30-days trial period. Not bad if you use multiple email addresses.
What I did during the trial period was to migrate just one (less important) email account, preferably used for receiving notifications from forum subscriptions. Once I get the hang of it, and spending some time on their tutorials and help sections, and reading reviews on websites and Youtube, I decided to migrate everything over and pay for it.