Why TLS email delivery is definitely important?
You probably already have seen such red lock displayed on an email received on your Gmail/Gsuite mailbox, with a message "[email provider] did not encrypt this message". This actually means this email has not been delivered through a TLS connexion.
If it's red, that's bad, right?
Yes ! Actually now, one of the good practices of the email industry, and very strongly recommend, is to deliver emails through TLS.
What that means ?
It means the sending SMTP server must initiate a secure transaction with the recipient server, and to create such a tunnel. All the datas passing via this tunnel are encrypted (basically the emails sent). Anybody trying to spy the traffic between sender and recipient will only see encrypted traffic with no option to decrypt it or alter it.
This technique is using web TLS protocol, based on strong asymmetric cryptography to secure the exchanges between servers. Most of the email providers are now using TLS to deliver messages, and those not yet delivering via this encrypted tunnel will possibly be soon penalized by major destinations.
Have you already checked if your email service provider were using TLS to deliver your messages? Check it out by testing your emails on Scan my Email, you will got few extra informations on the strength of the encryption protocol used to establish the TLS tunnel:
And feel free to contact your ESP to have infos on their roadmap if there is no TLS used, or if the ciphers used are weak or insecure.