Geary is an IMAP mail client for GNOME 3.
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Pro Straightforward and simple UI
Setting up accounts is a simple as putting in the users e-mail address and password.
Pro Fast and responsive
Geary is faster than most e-mail applications, upon starting the program or even just browsing in folders.
Pro Beautiful UI
Everything about the look and feel of Geary is a breath of fresh air after the clunkiness and ugliness of it's competitors.
Pro Threaded conversations
Threaded conversations means all subsequent replies are view-able underneath the initial e-mail.
Pro Native GNOME application
It looks and works like a GNOME application should, so it Just Works™.
Con No horizontal layout
There is no optional horizontal layout for those that want to be able to view their emails headers across the full screen.
Con No native Exchange support
Whilst possible if your Exchange server and account are using Davmail as a proxy, at the moment you can't natively add an Exchange account.
Con Not seen as a secure app by Gmail
Con Made for Gnome 3
It may look out of place in any other desktop environment.
Con No incoming message rules
Con No images in HTML signatures
Whilst it is possible now to add a HTML or plain text signature per account, there is no support for per images in the signatures. This is a must for when an employer imposes a set signature with images.
Con No proxy support
Seems like it can't retrieve email from behind proxy.
Con No PGP
No encryption available.