The Best Plugins to Supercharge Thunderbird
There are still plenty of great reasons to use a desktop email client
There's nothing wrong with webmail, but if you want complete control over how your mail is handled, filtered, organized, and even secured, a desktop client is the way to go, and Thunderbird is one of the best. This collection of plug-ins makes it even better, and even gives you some features you'd normally get in some of the other, paid email clients out there.
Thunderbird Conversations is one of our favorite extensions for Thunderbird
Filelink is Thunderbird's built-in cloud attachment feature
If you've rolled your own Cloud Storage service
As its name implies, Send Later allows you to compose a message and then schedule it to send whenever you want it to go out. Perfect if you want to send a message to someone on the other side of the world when you know they'll be awake to read it, or if you want to give yourself a little buffer time between writing an email and sending it to make sure you actually want it to go out the way you wrote it (perfect if you're the type who types angry, hasty replies). The add-on works with Thunderbird's built-in ""Copy to Outbox"" feature, and gives you the choice to send in 15 minutes, 30 minutes, two hours, or to specify a time and date for each message.
If you're familiar with the little arrows next to a Gmail message that shows you whether the message was sent directly to you, CC'd to you, or sent to a list that you're a part of, you already understand how they work in Thunderbird
If you're a fan of Markdown, the super-simple way of editing and formatting text
Lightning is a calendaring add-on for Thunderbird. Most email clients have some calendaring functionality built-in, whether it's Google Calendar and Gmail, Microsoft Outlook and its built-in calendar, or even Mail.app and iCal. With Lightning, Thunderbird gets its own rolled in calendar, and while it's not the most feature-rich calendaring tool, it's come a long way. Combined with the Provider for Google Calendar add-on, you get bi-directional access to your Google Calendar, so all of your Lightning events go right to your Google Calendar and vice versa. It's the next best thing to having Google Calendar right in Thunderbird.
It's not perfect-you only get access to one Google Calendar, and it can be a little buggy as the developers behind both extensions struggle to keep up with Thunderbird updates (which admittedly have slowed down) and Google tweaking Calendar behind the scenes. In any case, If you like adding appointments from your email right to your calendar, getting alerts in your email client for upcoming appointments, and responding to event invitations right from your email client, you don't have to give up Thunderbird in order to get the flexibility of Google Calendar. These two add-ons will get it to you. If your organization uses MIcrosoft Exchange instead, or you want compatibility with Outlook calendars, you can try this version of the Exchange 2007/2010 Calendar and Tasks Provider, which works with the most recent version of Thunderbird (the official, non-beta version of the same plugin does not).
Previously mentioned TorBirdy
It's important to note that TorBirdy anonymizes outgoing mail (and even read receipts, since they're essentially auto-replies), but like any Tor traffic, it doesn't really make you anonymous to your mail provider-there's no reason you'd want to be, you are checking your email after all. This is really for those situations where you want to send an email to someone and don't necessarily want the message headers betraying where you're located or what ISP you're using. TorBirdy also supports Enigmail, which we'll get to in a moment.
Enigmail is a security extension for Thunderbird that uses OpenPGP to encrypt and digitally sign your email messages. We covered it a long time ago
Development on Enigmail has largely stalled, but that's not too much of an issue considering is still works well. Regardless, there's a ton of documentation, forums and mailing lists for it, so if you run into problems, someone's likely already found a solution.
These plugins add some pretty beefy features to Thunderbird, some of which aren't available in fancier, paid email clients. If you're not ready to shell out for Postbox or Sparrow (even though it's been purchased by Google), and some of the other clients we've mentioned lately like previously mentioned