Hi, Toni
So, that message indicates that SmarterMail can't load the message from the .grp. Either the .grp is corrupt or your AV/Anti-malware scanner deleted the .grp or a specific message inside the .grp. (Though, generally, if AV detects a virus it will delete the entire .grp..so corruption is probably the issue.)
Make sure you have exclusions in place for C:\SmarterMail\Domains -- or whatever path you're using for the domains folder -- within your AV/AM software. That would remove those services as the cause. This includes making sure Windows Defender has the same exclusion, if it's not in place already.
If you're not running AV/AM, then the .grp was definitely corrupted somehow. So..no, it's not a security issue.
Derek Curtis
COO
SmarterTools Inc.
www.smartertools.com