When comparing Mozilla Thunderbird vs eM Client, the Slant community recommends Mozilla Thunderbird for most people. In the question“What are the best e-mail clients for Windows?” Mozilla Thunderbird is ranked 1st while eM Client is ranked 10th.
Specs
Question Ranking
Pros
Pro Free (as in freedom and beer)
Pro Lightning Calendar and Address Book integration
Pro Available on Linux, OS X and Windows
Pro Good filtering system
Mozilla Thunderbird offers a flexible filtering system with the ability to set flags and read/unread, as well as sort/assign to new mail directories. There are also numerous plugins available to assist in the filtering.
Pro Very reliable
Version after version, Mozilla Thunderbird works as expected.
Pro Consistent and involving a moderate learning curve
The app design of Mozilla Thunderbird has not changed significantly since its beginning, making its learning curve almost non-existent.
Pro Large assortment of plugins available
Mozilla Thunderbird offers a huge amount of extensions to expand the usability and options of the client.
Pro Tabs for navigation
Pro Send large files easy
Install pCloud plugin for Thunderbird and your files (upto 20 GB free storage included) will be received as download links
Pro Calendar inclusive
Pro Still actively developed
They've been developing and improving it consistently for years.
Pro The only email client which synchronizes Gmail contacts properly
Pro Many useful options
Such as sorting through categories, etc.
Pro Free and paid version
It is free for 2 accounts and as long as you don't use it commercially. Paid version comes with support and can handle more than 2 accounts.
Pro Doesn't replace your attachments with "winmail.dat" files
eM Client sends straight .pdf files for fax messages flawlessly.
Pro Beautiful interface, customisable to your needs
Pro It's simple to add new accounts
Pro Includes Quick Text
eM Client seamlessly integrated the Quick Text extension that was available on Thunderbird. This makes it easy to quickly auto-insert dynamic text, based on many variables, saving time and the need to tediously repeat manual text replies.
Pro It doesn't store all emails on the disc
It saves space, unlike Thunderbird.
Pro Chat included
eM Client includes integrated chat, so you don't need another application for chatting with your friends. All common chat services supported (Google chat, Jabber and others).
Pro Very fast
Pro A perfect companion to your Gmail or other webmail you might have
It provides (almost) complete Gmail functionality (calendar, tasks, chat, contacts), while offering advantages of an application: offline use, more e-mail providers in one place, very good design, support, etc.
Pro It is still the best in this class
But only because it has no alternative.
Cons
Con Terrible integration with macOS
Simple things - drag/dropping pictures doesn't work, for instance.
Con Development slowed to only focus on stability and security
Mozilla Thunderbird has slowed down its development of features to only focus resources on security and stability.
Con Subject lines can (temporarily) disappear from the list
This is sporadic behavior.
Con Serious bugs
Clicking on a subject line can bring up the wrong email.
Con Outdated interface
Thunderbird's interface is very outdated and unpleasant. It feels more like a Windows XP application than like a modern MacOS one.
Con Constant syncing errors
Con Not many options to manage IMAP folders
For example, all emails from Gmail will unnecessary synchronize.
Con Interface is too simple
Many options are too hidden and it takes at least 3 clicks to set them up.
Con Bloated
Huge size, high memory consumption, uses CEF (Chromium Embedded Framework).
Con Account options are not shown under right click
Con No conditional formatting
Con Only has one nice interface/skin
The other available skins has a 90s feel to them.
Con Terrrible in handing unread emails
Sorting of unread emails is very useless when compared to other email clients.
Con Poor integration with Yahoo Mail
Integrations with a Yahoo Mail account doesn't always work well in eM Client.