Lots of reasons besides MDM pointed out.
Security- no 3rd party server doing who knows what with your data. Sure Gmail app for a gmail account or Outlook for an Outlook/Exchange mail account, but most people have more than 1 type; and then what are they doing with your data? Only Apple's app can connect directly to the mail services. Yes I get email is not encrypted etc etc, but it doesnt mean you should open the door up either IMO.
Apple's Mail app wont disappear tomorrow like some apps have making you change your whole workflow when features of other apps dont compare.
Apple's Mail app wont start costing $50/year like Cloudmagic/Newton and Airmail forcing you to change your workflow.
Apple's Mail app is integrated into iOS at the system level and Apple is very unlikely to let you set default 3rd party apps in the near future.
The main free options left:
- I tinker with/beta test Outlook and its a good app and I doubt going anywhere (but missing a ton of basic stuff still like email formatting, draft syncing with the server, HTML signatures unless you use HTML code,cant search your calendar for some reason that makes no sense, etc). I think its still the best overall app though and just works. I hate hate hate that you cannot minimize an email to look back at something else like the Stock app or Spark; and have to dig back into the drafts folder to go back to it.
- The GMail app is meh at best and I dont use Drive (use Onedrive) and Google still doesnt let you use the Files app or even another cloud service to attach to emails, so that's instantly crossed off the list.
- Spark is pretty decent (and beta for them too) and the most features of any free client, but I question how long it will remain free as they just moved PDFExpert to $50/year FOR A PDF EDITOR APP (ridiculous) not even using server resources; so I highly question how long Spark will remain a free service honestly. It's also a bit too "cartoony" feeling for me aesthetically.
- Edison is just meh to me too (used to beta for them but got disinterested in the app and slow progress in features). It has some basic stuff like the assistant that I wouldn't use, but I hate hate hate the threading method they use (totally unlikes Gmail where one email is on top/below the other and you can expand/contract for context) and cant use it. And the whole caught with their hand in the cookie jar employees reading your email without any notification to the user, whether they fixed that or not and let you opt out now, turned me off to their company. Fool me once.
There is no perfect client IMO.
All of the toys are fun in 3rd party apps. But I can count the number of times on 1 hand that I have personally used Send Later or Snooze (a bad habit to get into anyway as I run an inbox zero method) in the real world outside of testing it out. And in iOS13 you can create a Reminder from email now (without Siri, the remind me of this rarely worked right in iOS12) with a link right back to that email no matter what folder you put it in, so that is basically the exact same thing snoozing.
Im sure some have a use for it but I dont, even running 3 businesses. If a customer/client emails at 8pm they get a reply then or the email typically. Im my line of business getting customers is all about clients getting answers then and there before they go elsewhere. I dont care if it looks like I only answer from 9-5 with send later, and actually looks better on me replying right away to clients from comments I get. If Im going to write the email then Im going to hit send it then, not set it to send at 9am.
If I need a read receipt for proof of something important being sent to prove the client got it, there are free services you just BCC to them and they track the read receipt for you on their site. They work far better than any in-app implementation I've used too. Sure you only get a few per month for free but for how rarely I use them it works fine.
