After restarting the computer I got the following problem:
When starting a new message in PMail, the message window is now positioned in such a way that the upper part of this window is hidden behind the top of the PM window.
How can I prevent this too high message window?
A restart of the computer did not help.
It is the first time I met this problem. I use v.4.80.
The problem occurs not only with
new messages, but also with replies.

I'm not sure I follow you. Are you referring to the message editor window? Pegasus Mail saves the size and position of its windows when they are closed, so move the displaced window to where it should be and close it. It will remember its new location the next time it is called.
Sometimes windows can be out of reach like when changing monitors. The best way to regain control over windows is to cascade them (e.g. Window > Cascade), re-align and close each to save, etc.

How about providing a screenshot? This forum allows uploading of pictures from within the edit section when replying.

Demo of too high message window at "Start a new mail message".
The same at "Reply".

As Euler suggested, Window > Cascade (or Window > Tile) should resize the window within the Main Pegasus Mail window. You and then resize and reposition it as you like. Best to do this with all other windows closed.

In this case, he can simply use Window > Enlarge (Shift+F4), as long as he clicks anywhere in the editor window to give it focus. After that, he can resize the editor window to suit his needs and close it to save the settings.

I "played" with F4 which resulted in a huge enlargement of the windows, but in such a way that the desired "start" window came within the boundaries of the PM window. Then I reduced it to my liking and I got what I wanted.
Thank you, Euler!

I'm glad I could help.