Hi, Support,
We found some of our mails sent to recipients are failed with error "553 DMARC domain authentication fail" who uses Messagelabs.com as SMTP server, but next time send to same recipient can be succeeded. Seems some IP of messagelabs.com are failed on SPF/ DMARC verification but the others can pass. Could you please advise who messagelabs.com support can help check it? Thanks.