Web, domains and email
MTA
Abbreviation for mail transfer agent
In short and in plain language
An MTA is a mail server.
It takes in emails and passes them on to other servers.
A domain can use a rule to ask that such servers send emails in encrypted form.
If you change your email provider, this rule should be updated in good time.
Definition
An MTA is a mail server that accepts emails and passes them on to other servers. MTA-STS is a policy with which a domain requires emails between such servers to be transmitted encrypted. When changing email provider, this policy should be adjusted in good time.