There are a number of settings you can do to adjust sending emails in Thread to your preferences. These can all be found by going to your initials/avatar in the top left and selecting Personal Settings.
Firstly, ensure you have your personal Email Signature set up.
*(If you are unsure if this email address was set up as Thread dedicated mailbox, check with an admin user in your company or see here for more details)
To ensure all document attachments you send by email are converted to a PDF document by default, have this toggled on in your personal settings.

Choose your preferred default email subject option when sending emails from a matter. This will auto-populate the email subject, however you can always manually edit the subject as needed.
Choose your preferred font when sending emails.
You're all set to start emailing from Thread! Find more guidance on managing emails in Thread here: Matter Emails - Overview