[oelmekki]

It seems to be, as far as I understand.

If that's actually the case, how do you handle the case where someone read a new mail (and thus, mark it as read) before app processed it ?

[jtmoulia]

The application uses IMAP UID/UIDVALIDITY values, typically along with IDLE, to identify new emails in a mailbox. These don't change when an email is read, so marking as read doesn't introduce any race conditions in this case.