Well, it depends. (Which is the usual answer to this sort of question!)
By default, Internet email is not at all secure; the underlying protocols like SMTP and IMAP are plain text and have no security.
However, sensible email providers will use an additional protocol, TLS, to encrypt the connection between your phone and their servers. Modern versions of Exchange use TLS by default; so does Gmail.
This secures the communications between you and the mail server. (There are some situations in which a bad guy can break TLS if they can intercept your network traffic and persuade you to accept a fake certificate. They're not common but a real threat and in particular your phone company can often do this fairly easily.)
More importantly, there is an aspect of email security that is usually forgotten; you can secure the sending part as much as you like, but you can't do anything about the receiving side. The person you are sending the mails to might not be using TLS; they might be very insecure.
In practical terms: email is secure enough for "regular use" if you have a reasonable email provider and are careful about not accepting bad certificates and avoiding dubious WiFi. However, if you have anything very sensitive to send, you should try and encrypt it separately before sending. (Which might be tricky on an IOS device.)
In a comment you ask if it makes any difference which client you use where you have an option, Apple Mail? Gmail or OWA apps? Web mail in the browser? These all use more or less the same methods to protect your mail from snoopers, so all are roughly equivelant in security from that perspective.
Google would say that using their Gmail app or the browser are slightly more secure, because only they support 2-factor, which is true. We always like 2-factor here - iF you don't have it enabled, go do it now!
However, because the second factor for gmail is "having your phone with you" it doesn't help much with the security when you are reading your mail on the same phone.