The envelope sender is communicated in the SMTP MAIL FROM command. It is the address where bounces are sent to. The receiver preserves this address in the Return-Path header when the email leaves the SMTP domain and is delivered in a mailbox.
For ESPs, it is common to use a common domain owned by the ESP in the envelope sender for all customers. For private systems, it is better to use a domain that is “aligned” with the From header domain. This is a requirement for DMARC compliance.
If your recipients use Outlook, Hotmail (and other Microsoft webmail services), or Gmail, they may see an indication in the email that it was sent by Pepipost on your behalf instead of just your ‘from’ name and address.
Following steps which include SPF and signing of DKIM helps to remove the ‘on behalf of’ or ‘via’ information. Even if someone displays via inspite of setting all, then it will be your domain only and not Pepipost.