Could some one help me to understand why the reply to email address was overwritten by the sender email address?
Don't understand what you mean by "overwritten."
The MAIL FROM (a.k.a. SMTP envelope sender) and From/Reply-To: headers serve different functions. Please explain your concern in more detail.
we had two different email addresse
from email address: firstname.lastname@example.org
reply to email address: email@example.com
Wondering why this happens?
You'd have to post the full headers of the received email together with a screenshot of the email headers set in the Marketo Email Editor.
A static string in the Reply-To: box should be the Reply-To: in the email that is actually sent out.
But you should definitely not use the expression "sender" here, when what you mean is "From:". "Sender" is something different.
I meant the from address.
we have received the alerts and automatic replies to firstname.lastname@example.org. Is this the problem with the email clients?
I have attached the email setup in mkto and the header of the email received.
I want to see the raw headers (not as interpreted/friend-li-fied by Gmail).
You should, however, search the Community as the question Which address receives automatic replies? has been discussed many times.