It’s not EE ‘adding’ those square brackets to the email, it’s how links/URLs are displayed in the preview window. I don’t get the same issue with EE’s emails, however I don’t own an Apple device so I can’t test on them.
Are the emails being sent as plain text? It could be the mail is being marked as spam, received as plain text and the client is interpreting the HTML for you but the preview shows the first element.
What are you using to send the emails from your site? Your host’s mailserver?
Test an email using a transactional email service and see if the problem persists, we have some listed here: