There was a change in MUI4 for AmigaOS4 and it impacts both YAM and SimpleMail. Here is the bug outlined for YAM: https://yam.ch/ticket/484
To borrow text from that bug:
Summary
When using SimpleMail .4 with MUI4 on AmigaOS4 and receiving emails, potentially the following error might come up:
MUI_Redraw() was called from task ... instead of the application's main task!
This is a violation of basic MUI rules!
Steps to reproduce
configure a multiple number of accounts in YAM and try to receive mails from these accounts several time.
retry to do the same with enabled preselection to better trigger the problem.
Expected results
The mails should be received without such an error/warning
This is due to a change a change in how MUI handled that error message. Seams in 3.9 it would only last for a fraction of a second thus no one noticed it before.