Question

I am working on a mail client application which syncs emails for a GMail account using the IMAP c-client library.

How can I get the most recent unseen/unread e-mails in the mailbox without blindly searching for all unread e-mails?

A mail search needs to pull all unread e-mails on every sync to the client but it is quite an expensive operation to perform on every sync.
Is there a better aproach to communicate to the client any unread e-mails which were not sync’d on the previous interaction with the server?

Thunderbird, for example, is able to sync unseen e-mails with some mechanism (possibly by doing a blind search for all unseen e-mails) as the IDLE command won’t notify
the client about them.

Is there some mechanism that can tell the client about unread e-mails that have appeared since the last sync?

Answer 1

There is an IMAP extension for Quick Flag Changes Resynchronization (RFC-4551). With this extension it is possible to search for all messages that have been changed since the last synchronization (based on some kind of timestamp). However, this extension is not widely supported – in particular not by gmail’s IMAP server:

* CAPABILITY IMAP4rev1 UNSELECT IDLE NAMESPACE QUOTA ID XLIST CHILDREN X-GM-EXT-1 UIDPLUS COMPRESS=DEFLATE

There is an informational RFC that describes how IMAP clients should do synchronization (RFC-4549, section 4.3). The text recommends the issuing the following two commands:

tag1 UID FETCH <lastseenuid+1>:* <descriptors>
tag2 UID FETCH 1:<lastseenuid> FLAGS

The first command is used to fetch the required information for all unknown mails (without knowing how many mails there are). The second command is used to synchronize the flags for the already seen mails.

This method is widely used. Therefore, many IMAP servers contain an optimization in order to provide this information quickly. Typically, the network bandwidth is the limiting factor.

If you are only interested in the UNSEEN flag, a UID SEARCH is probably the best you can do.