1
Sender vs. reply-to
The email comes from Picvelo's mail server, not from your mailbox. But the reply goes wherever a separate field, the reply-to, points, and that field holds your account's address. To the client, the whole thing looks like an email from you, with your brand.
2
When the "do not reply" note disappears
Picvelo adds a line about an automatic message to the footer only when it doesn't know your address. When the address is saved on your account, the note doesn't appear at all, because it would cut off the only working way to reach you.
The preview in the panel shows exactly what goes to the client, including this decision.
3
Emails that DON'T come back to you
System emails from Picvelo to you (notifications, reports, payment reminders) use our mailbox as the reply-to. In those, the do-not-reply note stays, and it's true.