When comparing Microsoft Outlook vs Posteo, the Slant community recommends Posteo for most people. In the question“What are the best email service providers?” Posteo is ranked 1st while Microsoft Outlook is ranked 10th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Good built-in calendar
The calendar includes agenda, day, and three-day views.

Pro Integration with calendar and tasks
Quick conversion of a received email to a task with the deadline.
Pro Fun to use
clean and smooth experience, very nice and i like it.
Pro Smart lookup
Smart lookup is a feature where the Bing search engine is bundled in the app to allow users to automatically search for relevant websites, information and images based on the word highlighted by users, without need to open external browsers. The results will be shown on the right pane of the window.
Pro Priority messages
Microsoft Outlook comes with feature called "Clutter" to filter out incoming email based on the priority of the message set by user. User will be alerted when the high-priority message is incoming, whereas the low-priority message will be put in a separate folder.
Pro Attach files from cloud
Microsoft Outlook allow users to directly attach an attachment directly from cloud services without need to download an entire file before attach to the intended e-mail.
Pro VBA macros
Pro Conditional formatting
Pro Cloud
can attach files and use cloud, very good.
Pro Numerous feature/advantages
Pro No ads
Posteo is ad-free with no ads when using their email services.
Pro Support for IMAP/SMTP
Posteo supports email clients such as Thunderbird and Claws Mail which will allow for greater control over encryption and user's private keys.
Pro No Personal Information required to register
Registration through VPN is allowed and no personal information is required, just a username and password.
Pro Very secure
Posteo does very well on Dismail's server list and it supports end-to-end encryption with PGP. This can be done in the browser or with an email client, and Posteo has a help section on how to set up encryption.
Pro Inbound encryption
You can select an option to encrypt inbound email from their server with a user-provided GPG key.
Pro Encrypted storage
You can select an option to encrypt all of your emails stored on their servers.
Cons
Con It's from Microsoft
Con Rather buggy
Con Crashes frequently
The only issue with Outlook 2017 is that when the Mail folder increases in size, it sometimes crashes for no reason it, causing loss of data.
Con Categorize email IMAP not good
Con Frustrating user experience
The "Clean Interface" approach means that normal and necessary user interface elements have been removed, such as certain scroll bars, and even the "replied to" indicator. This program becomes more frustrating with each new update.
Con No unique Inbox / global folders
Con No portable bundle available
Con No aliases
Outlook does not support email aliases other then outlook aliases
Con The UI is outdated
Con Only available for Windows
Con No custom domains
