Recipients can choose to opt out of receiving tracked emails. If a recipient has opted out, Mailsuite will prevent you from sending them a tracked email and will show a pop-up notifying you that this person no longer wants to receive tracked emails.
If you still need to send an email to that recipient, you can do so by turning off Mailsuite's tracking before sending the message. This will allow the email to go through without tracking, respecting the recipient's opt-out preference.