This is a side effect of the mail provider we use, which is Sendgrid. em.pressero.com is the hostname we use for our dedicated IP for sending mail. Gmail recognizes this in the headers and displays "via em.pressero.com".
This header will always stay as long as Aleyant's mail server is used. If you cannot tolerate this header, you must provide your own mail server and configure it in Pressero admin.
You can get more information on setting up the email configuration in Pressero admin in this chapter Ch. 006. Email Settings