Hello, i use Ritlab The Bat program more than 10 years (as you) and can resolve you issue.
Variant 1 - check file permissions and try seetings for shotcut for The Bat!
Variant 2 - upgrade The bat! to current version e-mail client
Variant 3 - convert mail messages base The bat! to e-mail*(eml/msg files) - as last varant...
May be any another variants.
Best regards, Andrew