I recommend adding a seed list to all broadcasts, which is manual and prone to being forgotten if Marketo users aren't properly enabled or the review process doesn't include it. I recently learned of the hidden BCC feature where an email address can be added to all Marketo sends. The reason I learned about it is it caused horrendously bad delivery rates. In fact, 100% of emails soft bounced on the first attempt, and then delivery rates were about 40% when all said and done. Perhaps that could be avoided by using a bullet-proof 100% delivery rate email address, and the one our client used was not, but I would caution you against using the BCC feature mentioned in other comments.
Thanks for sharing. I have also used the seed list method. There are some differences in the way I implemented them for each program.
I created a default program to hold my seedlists (Static Lists) and reference the static lists when I am building my audience wether it be on a smart campaign smart list or a default email program. This worked very well for smart campaigns and email programs.
In the case of an engagement program I just added the seedlist members to the engagement program but the program only had 1 stream. I can imagine it can get tricky depending on transitional rules and the complexity/quantity of streams.
There's nothing wrong with the BCC feature itself: it works exactly as described. Of course, not involving IT in the decision to receive all those messages is bound to cause problems, but it's trivial to set up a server that happily accepts millions of msgs from a known source with no rejections.
Agreed with Sanford... I'm sure the feature is workable. Our use case was some sort of convoluted SFDC email address that appears to cause 100% soft bounces. The problem was the email address rather than the BCC. I'm sure Sanford could set something cool up to process this properly, but if you go the BCC route, that inbox will get the same email 4k times if you send to 4k people, vs a seed list which will send it once.
I do agree that it's way too easy for someone to get Marketo to turn this feature on without vetting it w/IT (or in your case another hosted mailbox). Such is life when IT usually doesn't want anything to do with Marketo, not realizing they're implicated in everything Marketo does.
If you dedicate any real infrastructure to it, though, then it can handle all the messages you could send to it via Marketo (noting that Marketo doesn't send attachments or inline images, which is why you can be confident about that). Even an old PC with a big enough hard drive could spool all the messages. Now as for indexing and filtering the messages, that's another matter, but at least you could get them all onsite without a hiccup.
I might have identified an issue with this BCC... It seems that the BCC is sending a tracked version of the email, so we confirmed it registers soft bounces from the BCC (as well as the primary recipient). So say Marketo sends to Sanford with Dan BCCd, any action in either inbox will register for Sanford. To your point, if you set your BCC inbox up properly, perhaps you avoid all these problems, but seems to me the BCC should be an UNtracked version of the email to avoid the possibility of 2 inboxes taking actions on an email that should only be tracked to 1 person.
If it's the same Message-ID, it has to be tracked in the same way. From a compliance standpoint it would be worthless if the links were different.