New mail folder corruption and other problems occur once the new mail folder reaches in the 700-800 message range.  This has been a know problem in previous versions and appears to remain so in v4.61.  The solution is indeed to keep the new mail folder trim.  Many folks use the Main folder as the working folder but I struggle with that myself.  I prefer to use the new mail folder which means I must routinely clean it out by deleting messages or moving them to other folders.

I have just noticed when there are a large number of email in the in box of Pegasus the preview pane starts to get corrupted.

I noticed that pure text emails would start to display html text from other emails... THis started when I had about 1000 emails in my main inbox with the option to move read email turned off.

It got progressively worse as I started approaching 1200 emails. I also noted that this "corruption" would start to appear in other folders/preview panes. 

Note that the corruption would not show if the email was viewed at full screen.

I did try the Bear html install "fix" and this "bumped the limit from about 800 to the 1000 range.

I did turn off the remain in inbox after reading on exit (Tools Options Basic) exit Pegasus and restarted Pegasus. 300+ emails were moved into the folder and the preview  problems "disappeared" .