FastMail: Fast free or paid IMAP webmail with SMTP, POP
I rencently started using FastMail: Fast free or paid IMAP webmail with SMTP, POP & IMAP access, in its full account mode ($19.95/year) and I must say it’s awesome. It has IMAP access, you can define rules with sieve, it has a great spam detection system (in two months I’ve had only a false positive - with a score of 5.04 and my barrier is set up at 5.00 - out of hundreds of spam emails I had received).
I’ve also been able to transfer all my emails from other accounts only accesible by POP and Web without any problem.