I think this is answered in Can I ask for recommended websites?
No to websites; Yes to webapps.
Websites are about content. You read / write / look at or listen to content. Hopefully, the site has features that allow that.
Software is functional. You interact with it and want content to change through your interaction.
While some websites clearly behave like software (Google Apps), most do not (Wikipedia).
I'd say that a mail service is solidly a "functional" thing - you're creating content, moving it around, managing recipients, etc. Many modern email providers are web-based; asking for a provider supporting a specific operating system or set of features in a mobile app would also be fine by me.
Also tangentially related: Are web service recommendations off-topic?
I don't see why web services would be off-topic.
This is a software recommendation site. Whether the software runs on your phone, on your laptop, on your home server, on your rented cloud instance, on your company server or on someone else's servers doesn't change the on-topicness of the question.
And Including cloud-based services for software recommendations?:
In my opinion, it makes sense that cloud-based software (SaaS) recommendations be on-topic on this Stack Exchange website.