For the tagging part, since I found it sometimes has a problem even on the server side. For example, Gmail does not set the "Draft" flag well, and that's why I can not implement the edit draft functionality, as discussed in the following issue.

https://github.com/eclipse-ee4j/javamail/issues/345#issuecomment-431338396

I already reported the problem the GMail. And also some Chinese mail providers even do not handle read flag very well, if I mark a email as read and then mark it as unread again, the message still has "Seen" flag setted.

Maybe if I could find a reliable way in the future, I counld add the customized tagging an a feature in future release

Hi burnout,

Yes. It only support IMAP.

So the pro version does not contain Ads and can add more than three accounts. But the free version has ads and can add at most two accounts. But the rest of them are the same.

I complete started this from scrach, so it is not a skin on some existing mail client. I just use the javaee mail library for the imap/smtp part. All other things are implemented by myself.

Maybe you could give it a try for the free version first :).

You can read the full story at my blog:

http://m00nlight.github.io/android/2018/10/20/story-behind-omail-an-email-client-with-automatica-mailing-list-managment-on-android

What's the difference between free and pro?

I assume IMAP support only?

Tell us more about the code and development. Is this a skin + added features for some existing mail client or did you write everything yourself? As in, does it use some existing Android APIs for IMAP and SMTP etc. or did you make your own implementations?

From the article:

It does not have the feature like custom tag in Opera Mail, because the custom tag can only store locally,

A lot of IMAP servers support IMAP keywords. It might be possible to use them to tagging.