Unix server administrator needed asap. Centos 7. Make sendmail secure smtp relay to 1and1 smtp server or secure sendmail to stop email bounces

Ok, I need a high quality proven unix systems administrator for what should be a relatively straightforward task. At the moment not all emails from my ecommerce store are being delivered to clients. Gmail for example is refusing delivery of the emails. this is one of the refused emails when viewed in Alpine client on the server. '< 550-5.7.1 [2a01:7e00::f03c:91ff:fe31:302c] Our system has detected that this <<< 550-5.7.1 message does not meet IPv6 sending guidelines regarding PTR records <<< 550-5.7.1 and authentication. Please review <<< 550-5.7.1 [login to view URL] for more information I have a 1and1 Imap email account and I want to relay mail through that ideally (it must be port 587 TLS) and so I need sendmail configuring securely .So the setup has to comply with IPV6 mail sending guidelines. Alternatively we can use the servers own sendmail as a secure relay, firewalld port will need opening for port 587 also for either option. At the moment it is relaying through smtp port. Security is important. This needs PTS records setting up, I can add those when needed to DNS Only experienced trustworthy sys admins please with a lot of feedback who can do the task quickly. thanks '