K-9 vs FastMail

K-9 is available for free and licensed under the Apache License version 2.0 with source code available here.

K-9 mail does not use too many system resources when using the app, which allow it to be fast and lightweight.

It's possible to complete encrypt email communications via PGP using K-9.

Works well with locally maintained IMAP server.

On a tablet the UI looks a little different but given the extra real estate is logical.
Options to enable UI widgets a plus, can be enabled on a tablet and disabled on a phone.

There is a fork of K-9 available on Google Play that uses Material design, improves the interface a lot.

Threaded email support is better than most.

Depending on your point of view, requiring to pay for Fastmail is actually a positive aspect of the service. This way you know the company is at least making some revenue (they have been around since 2010) and you know who the customer is. Pricing is as follows (go here for their on-site pricing info).

As you are their customer, Fastmail provides human support.

Gmail has a tendency to dumb down the interface (try to sort by sender). Fastmail gives full control over email, just like a "traditional" client would.

Supports two-factor authentication, one-time passwords and fully encrypted connections.

The Fastmail interface is fully responsive across all browser sizes.

As Fastmail does away with all the extra bells and whistles of Gmail it is super fast to interact with. It also has faster IMAP and SMTP performance than Gmail.

You can write your own Sieve scripts for filtering/rerouting email.

Like gmail, you can "Pin" emails to the top of your inbox to deal with later with either a checkbox or the keyboard shortcut. Like Gmail you can also both archive and delete emails.

Get your old email out of Gmail easily with their migration tool.

On writing this Fastmail has Basic, Normal, Aggressive or Customisable spam filtering which can be selected in the Advanced settings.

Each user has an individually trainable bayes database to improve spam filtering. You can train your personal bayes database by selecting some messages and using the Report spam and Report Non-Spam options from the Action menu on the Mailbox screen.

For IMAP users who don't use the web interface much, you can specify folders as spam/non-spam training folders on the Folders screen. Then just move messages into those folders as appropriate to train your bayes database.

For your personal bayes databases to be active, you have to train it with at least 200 spam messages and 200 non-spam messages. If you haven't trained enough messages, then we use a global continuously updated bayes database against your incoming messages, which helps detect spam/non-spam, but isn't as good as a personally trained one.

Instead of using plus addressing (e.g. foo+bar@email.com), FastMail offers the ability to receive mail through subdomain addresses (e.g. foo@bar.email.com).

Fastmail allows saving attachments and other files into their own file storage system for easy retrieval later.

FastMail has full support for folders, unlike Gmail where you only have labels.

Fastmail automatically filters your email into:

• All
• Unread
• Personal
• Notifications
• Mailing Lists

With identities and aliases, you can set up a completely new email address that can be managed from your account, without paying for more. As of time of this writing, FastMail offers up to 500 identities and 600 aliases per account, regardless of plan type.