In recent years Apple's innovation on the desktop has come to a halt. The Mail app has been severely neglected and really failed to deliver some of the features that are absolutely necessary if you're working in a professional environment. When the 'modern' layout was introduced to Mail I thought it would be a welcome change but instead I switched it right back to the Classic layout layout immediately. That wasn't the only feature or lack thereof that bothered me with Mail but there weren't many good alternatives so I stuck with it, until now.
So lets see my top five reasons for switching to Outlook and at afterwards I'll talk about the little quirks I found after the switch that I wish Microsoft will address in the future.
#1 - Office 365 compatibility
No matter how hard you try, some Office 365 implementations will not work with macOS Mail. I found this the hard way when a customer of mine switched to O365 and all my email stopped working. Apple does support Exchange on the Mail App, but with O365 a company can choose to further restrict how emails are accessed - in this case a typical Exchange account no longer works.
With Outlook this was not a problem. All I had to do was login to the O365 account and it synched all the emails to the server no problems. My other IMAP based accounts worked perfectly with Outlook as well so that was a great bonus!
#2 - Search that actually works
I really hated how Mail implemented search functionality. I never seemed to be able to find what I was searching for. Whenever I saw how some of my work colleagues used the 'Advanced Search' with Outlook I got a little jealous.
Outlook search is amazing, you can use multiple filters, search within the current folder or all of your mail, look for flagged items or categories, etc. The way a search query is created makes sense in Outlook, it's straight forward and it just works. In Mail this was never so easy even when using fuzzy search - and no I did not want Spotlight indexing my emails so that was disabled.
#3 - Look and feel
Compared to Mail, Outlook actually looks like an email client. I am not certain what Apple was going when designing the Mail UI, but it was not focused on usability. From the way the ribbon is laid out to how the Inbox is displayed everything in Outlook makes sense. I especially love that emails are separated into Today, Yesterday, etc. Unread email actually stand out in Outlook by showing up as a different colour and being bolded, not like in Mail where they are just bolded and tend to blend in with the rest of the email.
#4 Extensive Formatting Options
Formatting in Mail is extremely limited. In Outlook it's possible to format text almost to the level as it is in Word. Highlighting text - no problem - Mail can't do that! The formatting toolbar is easy to use and unlike Mail provides all the features you would want to use right there in front of you, no need to bring up the Fonts dialog box or anything arcane like that.
#5 Status Bar
A status bar that actually gives you the current status - that's what Outlook has. Connection information, number of emails in the current folder, send/receive progress - it's all there. Mail's temporary status during email send/receive doesn't measure up to this at all.
Not everything is perfect in Outlook however. There are some quirks I encountered when moving from Mail, they were not a major hinderance and definitely not a show stopper, but I do wish these aspects get addressed in the future...
Print scaling - In Mail it's possible to set the scale at which an email is printed/exported to PDF from the Print dialog box. With Outlook you have to do this via the Page Setup dialog. It's a little inconvenient and Outlook tends to forget this setting, and it wasn't exactly easy to find, but it is there.
Quoting of pasted text - I've not figured out a way to do this in Outlook automatically. Manually indenting works but this is an extra step. In Mail you can just use the 'Paste as Quotation' option. No big deal.
Toolbar not customisable - The toolbar/ribbon is great but there is no way to customise it. Not a massive deal but it would be nice to rearrange some of the buttons or even remove ones that aren't useful for you.
Calendar not compatible with CalDAV - This one is probably the biggest annoyance. The built-in calendar in Outlook isn't compatible with the Calendar app and it doesn't allow you to add additional CalDAV based calendars. This means I have to use both the Calendar app and the Outlook Calendar at the same time.
Same thread emails not highlighted in Inbox - In Mail when an email is selected all of the other emails in the same thread are highlighted, not in Outlook. However, opening an email and clicking the Conversation button shows a list of all emails in that thread, just an extra step and once you're aware of it, it becomes second nature.
Will I return to macOS Mail? Probably not, at least not while I am able to use Outlook. Unless Apple comes out with a whole new Mail app that can be used on a daily basis for professional work I don't see my opinion changing.