When comparing Geary vs Sylpheed, the Slant community recommends Geary for most people. In the question“What are the best native e-mail clients for Linux?” Geary is ranked 4th while Sylpheed is ranked 16th. The most important reason people chose Geary is:
Setting up accounts is a simple as putting in the users e-mail address and password.
Specs
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Pros

Pro Straightforward and simple UI
Setting up accounts is a simple as putting in the users e-mail address and password.
Pro Native GNOME application
It looks and works like a GNOME application should, so it Just Works™.

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 Lightweight
Pro Threaded conversations
Threaded conversations means all subsequent replies are view-able underneath the initial e-mail.
Pro SQLite DB for email
It uses SQLite DB for storing email messages from all accounts in one single DB file.
This makes it portable as in you can just copy one SQLite DB file and move your offline mail content to another PC/location. Also, one SQLite DB file for all messages means it is incredibly fast.
Pro Work in background
Pro TNEF support
Support for Outlook-specific email attachments (TNEF).
Pro Reliable
No issues with stability or stuff going missing.
Pro Comprehensive settings
Pro Very very lightweight
It is simple and very lightweight where claws mail added a lot of bloat like themes and plugins slypheed keeps it simple.
Cons
Con No PGP
No encryption available.
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 Made for Gnome 3
It may look out of place in any other desktop environment.
Con No incoming message rules
Con Too basic
Very Basic, cannot format date, cannot create new folders - What!!
Con Not seen as a secure app by Gmail
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 Development is pretty dead
No real development since October 2017.
This should be changed from a "Con" to a "Pro" as the Librem 5 smartphone has Geary as the default email application. There is ongoing development happening as can be seen at https://gitlab.gnome.org/GNOME/geary/commits/mainline
Con No Pop3 support
Con Bad HTML rendering
The HTML rendering of email messages is not full fledged. Some messages can appear a bit off compared to their appearance on browsers
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 Its bloated
IT requires many GNOME dependencies or it will be unusable on the most non other X11-desktops
Con No proxy support
Seems like it can't retrieve email from behind proxy.
Con No encrypted mailbox
Likely hasn't been done by anyone since Calypso/Courier back in the 00's.
