You can send Mass mail from eM Client with a PRO license.
Mass mail is used for messages that are sent to multiple recipients but each one will get their own copy of the message (with no addresses in CC or BCC). You can read more in this article: Undisclosed recipients and how to send mass email in eM Client

It is also the correct way to send messages to numerous recipients if you do not want them to see each others' addresses.

To use this feature, add the recipients or use a distribution list from your Contacts folder in the To: field.

Instead of pressing ‘Send’ navigate to Menu > Message > Send as Mass Mail in the new message window.

The option is also available under the Send button drop-down menu (Send > Send as mass mail).

You can also use variables in Mass Mail emails that will replace information in your email with the recipient's details based on what you have saved to the contact's details in your Contacts data.