How to remove / delete eMail header

How can I delete eMail header which are shown in eM but have been opened and deleted in e.g. Gmail or others?

When I first open Gmail then read and delete an eMail after then I  open eM Client in order to receive further eMails in eM Client, those deleted eMail header were displayed in eM but can not be removed or deleted or handled anyway.

How can I get rid of those remaining “bodies”??