I have written a number of articles about Alibaba Cloud DirectMail on my website. These may help you understand the product (service). Alibaba Cloud Articles You can also find these articles on Alibaba's Blog website Alibaba Cloud Blog
There are two types of encryption when it comes to email. Envelope Encryption, meaning that the email itself is encrypted and Data in Transit, the phase when emails are being delivered from on email server to another.
Alibaba DirecMail does not provide Envelope Encryption. This is a feature of the email client to encrypt an email. DirectMail will process an email encrypted or unencrypted.
Alibaba DirectMail does support encryption for Data in Transit to their servers. This means the path from your email client to their server. This is optional and you enable this by selecting an encrypted port such as 465.
The last item to think about is email data transfers between email servers. An email can go from the origin server thru several servers on its way to the destination inbox. You have no control over this process. Most enterprise class email systems will attach email headers that you can review to see if encryption was used as part of the transport protocol between servers.
If your goal is to encrypt your emails, select an email client that supports email encryption. This sounds simple, but in the real world email encryption is a pain to manage. I instead recommend creating a text file with your secrets and then encrypting the file. Attach the encrypted file as part of your normal unencrypted email.