Maybe https://www.mail-tester.com/ https://www.mail-tester.com/ would help. Also https://mxtoolbox.com/diagnostic.aspx https://mxtoolbox.com/diagnostic.aspx .

Maybe https://www.mail-tester.com/ https://www.mail-tester.com/ would help. Also https://mxtoolbox.com/diagnostic.aspx https://mxtoolbox.com/diagnostic.aspx . If neither of these tools highlight any issues it's pretty strange indeed.