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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.
Con Constant syncing errors
Con Terrrible in handing unread emails
Sorting of unread emails is very useless when compared to other email clients.
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.
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 Poor integration with Yahoo Mail
Integrations with a Yahoo Mail account doesn't always work well in eM Client.