Hello alibabà,
Thanks for reaching out to Apple Support Communities.
To help us understand more about the issue, we have a couple of questions.
Would you prefer to send non-encrypted emails all the time?
Encrypted emails require the recipient's certificate in your keychain. Do you experience any issues when sending encrypted emails?
We have addition information about sending digitally signed and encrypted emails. Take a look below:
A digitally signed message lets your recipients verify your identity as the sender; an encrypted message offers an even higher level of security. To send signed messages, you must have a personal certificate in your keychain. To send encrypted messages, the recipient’s certificate must be in your keychain.
1. In the Mail app
on your Mac, choose File > New Message.
2. Move the pointer over the From field, click the pop-up menu that appears, then choose the account for which you have a personal certificate in your keychain.
A signed icon (containing a checkmark) is shown in the message header and indicates your message will be signed when you send it.
3. Address the message to recipients.
An encrypted icon (containing a closed lock) is shown if your keychain contains a personal certificate for every recipient. If you don’t have a certificate for every recipient, click the encrypted icon in your message; an open lock replaces the closed lock, indicating the message will be sent unencrypted.
This information is from Sign or encrypt emails in Mail on Mac - Apple Support
Regards.