Thunderbird is free, and is getting a lot better at it's uniform appearance across platforms.
It auto-configured on all our test devices without issue and so far is proving to be a valid contender. Price point always helps. Saying that SmarterMail shouldn't work to support multiple clients just feeds the beast of vendor/client lock in when it comes to features and compatibility.
The best thing that can happen with Thunderbird is people use it, and actually participate in the bug-reporting process. We're dealing with that now with the XMPP connector.
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