That said, we never say never, and may be able to develop a version of Boxer on iOS that would work for it. However, we would need a good deal of feedback telling us it's wanted. You can let us know that you want it in our feature request forum here: http://support.getboxer.com/knowledgebase/articles/316551-support-for-pop-or-pop3-accounts
Pop3 is supported with Boxer for Android, since this functionality was brought over through the acquisition of Enhanced Email. There are still limitations with Boxer functionality for these accounts, but you are able to connect Pop3 accounts to Boxer for Android.