If I am using retention polices (via Security & Compliance Center) for archiving mails and if I need to relay important mails from on premise systems to customers over Office 365. Will they be archived too? I don't think so, or? Is there a solution to archive relayed mails either?
No, the retention policies and other compliance features only apply to mailboxes stored in O365. Depending on how you send the messages, you might be able to configure a transport rule to BCC those messages to some mailbox or use some other form of journaling.
Best Response confirmed by
Patrick B (Contributor)
Okay, thank you. Journaling does not work in this case because the journal mailbox in a journal rule in Office 365 must be external (and therefore cannot be an Office 365 mailbox). We try to use a transport rule, even though I'm not quite sure what the legal department says about it, because this will change the original e-mail.