Polymail Pro provides a solid desktop and mobile email client that includes a suite of killer productivity features including 1) defer to future date, 2) remind if no reply, 3) send later, 4) calendar invites, 5) open receipts, 6) link click & file download notifications, and 7) templates. It also gives the option to use the Gmail keyboard shortcuts many users have memorized.
The downside of the return if no reply functionality is that brand new messages often get mixed in with messages that have been returned because the recipient didn't reply, so I wish that I could filter out new messages from returned messages. Currently, it only allows me to filter read vs. unread messages, but it treats returned messages as unread. Perhaps they need an Inbox (new) and Inbox (all)?
Also, the mobile app could be vastly improved if it allowed me to swipe forward and backward to power through the inbox. Now I have to click to return to the inbox and click again to read the next message. It doesn't sound like a lot, but it adds up when you only have 5 minutes to check email.
Lastly, they have a weird bug that doesn't recognize some HTML links in some emails. It's not a deal killer, but I wish they would squash it as it has been going on for a year.
The contacts sync has been buggy, but it is improving. If you make a type-o when entering an email it is impossible to remove that bad address, and polymail will suggest it in their auto-fill options so it is easy to send it to a bad address again.
Every so often they have difficulty adding attachments.
The feature set provided by Polymail helps me make sure that nothing slips through the cracks. It is easy to forget that someone owes you a reply, so the return if no reply makes sure I remember to follow up. I use defer to future date to get something out of my inbox until I am ready to deal with it. The send later feature allows me to reply to email at all hours and have it sent during normal business hours, so people don't know I am working at midnight. I can also use it to send myself and others future reminders.