When an inbound action log says ignored by filter, it could have been ignored by one of the many filters configured in the platform.

email address filters : https://.service-now.com/sys_email_address_filter_list.do?sysparm_userpref_module=5254cff2571313007f004758ef94f943&sysparm_clear_stack=true

system address filters : https://.service-now.com/sys_email_system_filter_list.do?sysparm_userpref_module=07055f1773f8001086592f163cf6a70b&sysparm_clear_stack=true

notification filters : https://.service-now.com/notification_filter_list.do?sysparm_userpref_module=54d297ab0a258103004f42868463cb88&sysparm_clear_stack=true

email filters : https://.service-now.com/sys_email_filter_list.do?sysparm_userpref_module=3c25270b0a0a0b2400f9efd158cf631b&sysparm_clear_stack=true

One common example is spam filter which is an email filter, a spam can be identified by looking at the header of email

X-ServiceNow-Spam-Flag:YES

X-ServiceNow-Spam-Score:x.xx

if spam flag is set to yes it implies that the email is spam and hence ignored.