Just quoting part of the answer (What is MTA-STS?)
MTA-STS, which stands for Mail Transfer Agent Strict Transport Security, is an email standard that secures inbound email and prevents attackers from exploiting a weakness in standard SMTP security. The MTA-STS standard, at its core, is a combination of having all of your email servers using Transport Layer Security (TLS), having valid publicly-trusted certificates for those servers, a published DNS record, and a TXT file. MTA-STS, once implemented, actively enhances security of inbound email to your domain from attackers looking to intercept unsecured emails.
summary: you should set this up and it's quite easy actually.