MXToolbox reports DKIM verification failures due to a multitude of reasons. These include potential glitches within MXToolbox itself, DNS record errors such as syntax mistakes or incorrect key placement, whitespace or line break issues within email headers or body, DNS propagation delays after updating records, DKIM selector mismatches, invalid characters in the DKIM record, problems related to subdomain DKIM configuration, conflicts arising from multiple DKIM records, signature manipulation during transit, incorrect key retrieval, signing process problems, and even exceeding DKIM keysize limitations. It's essential to verify results with other tools, check raw email headers, validate DKIM syntax, ensure the correct selector is used, verify proper subdomain configurations, and allow ample time for DNS propagation.