Hi, I have a question. I noticed in the admin panel it says you have to send mail using the box.
If I add my email to default iOS app will I be able to send emails from it just fine or will they be marked as spam to everyone?
You setup your account on the iOS mail app, using the SMTP of your box and you should be fine. The marking as spam has more to do with having the appropriate config of the box than with the use of an iOS app. This includes things like having PTR reverse DNS set, appropriate SPF, DKIM, DMARC, etc.