When comparing Business Calendar vs FastMail, the Slant community recommends Business Calendar for most people. In the question“What are the best calendar apps for Android?” Business Calendar is ranked 6th while FastMail is ranked 14th. The most important reason people chose Business Calendar is:
Business Calendar offers configurable widgets in many sizes for displaying Days, Weeks, Months or Agenda views on a user's home screen.
Specs
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Pros
Pro Widget support
Business Calendar offers configurable widgets in many sizes for displaying Days, Weeks, Months or Agenda views on a user's home screen.
Pro Free ad supported version available
Business Calendar comes in a free ad supported version.
Pro Smooth zoomable user interface
Business Calendar has a user interface that supports scrolling and pinch zooming.
Pro Search function
Business Calendar has a search function that allows searching the calendar for dates and events.
Pro Integrates with Google Maps
Users who input locations can use Google Maps from an event to find the directions and location.

Pro Move Calendar events from one Calendar to another
The only app that has the ability to move an event to a different calendar if it was accidentally created in the wrong calendar. This app is an essential addition to any device and for anyone with multiple calendar accounts.
Pro Events can be created using the share menu
Pro Can manipulate with multiple events at a time
The app allows selecting multiple events and move or delete them together.
Pro Traditional revenue model increases likelihood of personal data privacy and safety
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).
Pro Human support team
As you are their customer, Fastmail provides human support.
Pro Offers more control over email
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.
Pro Secure
Supports two-factor authentication, one-time passwords and fully encrypted connections.
Pro Fully responsive interface
The Fastmail interface is fully responsive across all browser sizes.
Pro Fast
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.
Pro Custom sieve scripts
You can write your own Sieve scripts for filtering/rerouting email.
Pro Simple email actioning
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.
Pro Offers IMAP migration tool
Get your old email out of Gmail easily with their migration tool.

Pro Adjustable spam filter
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.
Pro Offers subdomain addressing
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).
Pro Integrated file storage
Fastmail allows saving attachments and other files into their own file storage system for easy retrieval later.
Pro Proper support for folders
FastMail has full support for folders, unlike Gmail where you only have labels.
Pro Intuitive default filters
Fastmail automatically filters your email into:
- All
- Unread
- Personal
- Notifications
- Mailing Lists
Pro Identities & aliases practically gives you a new email that you can manage from a single account
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.
Cons
Con Poor visual design
The design of the app looks a bit dated, something out of the early 2000's. While this does not impede the usefulness of the application, it still is a sore point when compared to some of the other popular calendar apps available on Android.
Con Widgets don't show week numbers correctly on some devices
Widgets on certain devices have been showing week number incorrectly for more than 2 years. The issue hasn't been addressed and there is no sign that it will be.
Examples from S4:
Con Poor support
The developers provide virtually no help with issues.
Con Australian law enforcement
The company and all email hosted by then is subject to Australian laws requiring sharing with law enforcement. These provisions are weaker than those provided by European GDPR.
Con Lacks folder notifications on official mobile app
Con Expensive
The most popular domain registrars will provide you with both multiple email accounts and a domain name at a lower price than FastMail charges for only 1 email account.
Con Servers are located in the US
Con Not free
