webmail bounced back if the recipient is a gmail account

I have a Wordpress website hosted in AWS Lightsail/Plesk. I found all the outgoing webmail are bounced back if the recipient is a gmail account. I consulted with the AWS support and they replied saying that: "I also looked into the error you faced while sending the emails to Gmail. You faced the error '550-5.7.26', which on Google's support website lists the reason for denying your request as unauthenticated, i.e. it didn't find an SSL certificate to be authorised to send emails to Gmail. You need to configure your email service with an SSL certificate with v1.2 or higher to bypass this error" However, another hosting company said: "We believe it is not related to the SSL-Key. Maybe you can check with your server service provider to see if their email server support TLS1.2, if yes, you may ask them to enable it. If not, you may consider to upgrade your server OS as CenttOS 5 not support TLS1.2 feature. " So I am looking for someone who is really experienced and have absolute confidence in this field to tackle the problem. Anyone who had experience in fixing exactly the same problem is most welcome.