The text of the emails you send using LSP.expert can be changed in Settings > Document and email configuration > Emails tab of the configuration in question. You can format it as you like (bold, italics, etc.) and define a different text for your invoices, invoice reminders, quotes, purchase orders and credit notes.
You can also customize the content of your emails using variables. In the Emails tab above, you will find a little button called "help on variables" for each email text. When you click on it, you see all the variables you can use:
You can insert those variables anywhere you want in your emails and the system will adapt them with the correct info when sending your email.
When you're done, click on Save.