When comparing Boomerang vs Right Inbox, the Slant community recommends Right Inbox for most people. In the question“What are the best GMail addons or services to send emails later?” Right Inbox is ranked 2nd while Boomerang is ranked 4th. The most important reason people chose Right Inbox is:
Natural language form field doesn't support time zone yet, but the methodical date, time, optional time zone input field will.
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Pros
Pro Support for time zones
It's easy to send emails in a different time zone using the standard time zone abbreviations such as: PST, EST, PDT, EDT etc.
Pro Has iOS app
There is a full-blown iOS app for managing or creating emails.
Pro (Partial) timezone support
Natural language form field doesn't support time zone yet, but the methodical date, time, optional time zone input field will.
Pro Natural language input
Pro Available on major browsers
Available on Mozilla Firefox, Google Chrome, and Safari as addons.
Cons
Con License is per Gmail account
If you want to use Boomerang with multiple Gmail accounts, you will need to pay and manage subscriptions for each Gmail account.
Con Difficult to edit emails once you've scheduled it to send later
Half the time you can't edit an email once you've scheduled. It's a very buggy function. The more reliable way to do this is to cancel the scheduling, go back to the drafts/inbox etc. Edit it as a normal draft then reschedule it. It's quite the hassle. For a paid product it shouldn't be this hard.
Con Webapp doesn't let you schedule a draft you've already made
The web app can't schedule pre-existing emails you have made in drafts. You can only schedule entirely new email content making it really awkward to have to copy and paste in a mobile app.
Con Only useful on desktop web browsers
This service won't work on mobile devices.