Many of us see the "Not responding" verbiage in Pegasus Mail window title bars while Pegasus Mail is working.  My guess is that it is generated by the OS when Pegasus Mail is busy stops communicating with the OS for a period of time (a way too short period IMHO).  This started with the introduction of v4.70 when some processes started taking longer than they had.  This gives the appearance of a hang of Pegasus Mail and the associated display of the "Not responding" verbiage.  It only affects some of us and, AFAIK, the cause has yet to be identified.  If this sounds like what you are experiencing then there isn't a solution.  Only patience.  A "wonky" internet connection could exacerbate the issue.

Also, if the Win10 machine is an upgrade from Win7 do a reinstall of Pegasus Mail on top of the existing one (backup of the mailbox directory(ies) first).  We are suspicious that the Win10 upgrade process and the recent upgrade to the Anniversary edition sometimes adversely affect a Pegasus Mail installation.

Long-time PMail user here. Lately hanging, unresponding issues have been driving me (and my hubby) nuts. Partly the problem has been due to our internet connection being wonky, but since upgrading to a new gateway and having a direct line installed to the modem/router, Pegasus has been experiencing more problems than ever. It times out constantly and stops responding when attempting to download message headers. No problems with sending emails. Any suggestions welcome. I am using Win 10 (hate it); hubby's computer still runs Win 7.