What are the steps to reproduce your issue?

Setup 1:

Outgoing Mail-Server: default (odoo)

Domain: default (mydomain.odoo.com)

Setup 2:

Outgoing Mail-Server: smtp-relay.gmail.coma

Domain: mydomain.com

What is the current behavior that you observe?

Setup1:

I only get notifications for bounced e-mails when sending without attachments. When a file or report is attached I don't get a notification for the bounced e-mail.

Setup 2: 

Guide: https://www.odoo.com/documentation/user/12.0/discuss/email_servers.htm

tells me to create the forwards catchall@mydomain.com and bounce@mydomain.com to their odoo aliases.

works well for catchall because the catchall alias is always the same. Only the subject is changing.but the return-path (bounce alias) set to every outgoing message by the odoo system is different for every sent message: e.g . bounce+15-sale.order-48@mydomain.combut I do only have the forwarded alias bounce@mydomain.co as described in the guide. To get this working I think the variable part (15-sale.order-48) has to be in the Subject?

Odoos incoming server can handle it but I don't know how to setup my outgoing server to forward all these different aliases.

What would be your expected behavior in this case?

Setup 1:

Notifications also with attachments.

Setup 2:

Guide how to do that if possible.