You can send photos to clients and others through email. Because sending original file sizes may be too large to send as an attachment, Photo Mechanic offers options to resize and compress photos before they are sent. You can also add a watermark.  

Go to File> Send Photos via email... (or shortcut Shift-⌘-E)

You can also right-click on an individual photo in a contact sheet and choose to Send photos via email...

You can type in an email address or use the button next to each address field to select from your default address book. Messages will be sent from your default email account, for example, Mail on macOS. You can change this by using the Send email via ______ dropdown. 

The Subject: and Body: fields can be customized by using variables. To save time, save a snapshot of your default email messages and take advantage of User/Client variables. If you send multiple photos in one email, only the variables from the first image in the series will be used. 

Create a separate email for each photo: Use this option if you are sending full-size photos to avoid your or your client's email servers with attachments that are too large. You may still need to resize or compress the photo to send it by email. 

Note: Sending by email only sends JPEGs. You can specify whether you want to generate those JPEGs from the existing JPEG preview or the RAW image, but the photos attached to the email will be JPEGs.

Send Photos via email can use several popular mail clients on macOS, but it requires you to enter your mail SMTP information on Windows.