MARKETING in the subject line for email replies

[Anonymous]

We noticed that sometimes when prospects reply to Marketo emails, the subject line gets appended with [MARKETING]. For example:
Original subject: Imagine editing Office files, without Office
Replied to subject: RE: [MARKETING] Imagine editing Office files, without Office
Also, when you look in the email they actually replied to, it looks like the subject line they received was: [MARKETING] Imagine editing Office files, without Office

This makes it look like a large spam-type blast and not a personalized email. Has anyone seen this before? If yes, what's causing this and is there a way to fix it?
Thanks!
Liz

[Anonymous]

It's probably being caused by a filter either at the company email domain level or at the individual person's mailbox level. I'm not sure if that exact syntax points to a particular tool, but others here might.

In general, there probably isn't a lot to do except all the normal things you do for good deliverability and ensuring you engaging content.