How often do HA get synced?

Using 50.1 firmware in a 64 bit HA environment we have an issue with the spam mails.

There are listed on both units but if we click the mails they are only available on the first unit, the other unit seems to missing the mail itself.

The mail content is copied to the client unit with the synchronization, which occurs every 2 hours. The mail headers are copied in real time.
For this reason it appears that the content is empty. It only happens for the mail that came into the master unit after the most recent synchronization.