Emails sent by Marketo are being blocked

Deliverability management is a huge topic and can be very tricky as there are many different factors at play, just to name a few:

- the health of your database - follow Josh's advice above and make sure you don't email invalid/inactive leads

- the content of the actual email - spam filters will automatically flag emails that look spammy: spammy words, e.g. free, promotion, discount, offer, even $ sign; ALL CAPS in the body of the email and especially in the subject line; entire email being an image; too many images; broken HTML, etc., might get your email flagged as spam and lower you deliverability rates (this often explains why some of the email you send have much lower deliverability rates than others). There are tools out there that allow you to test your email inbox placement based on the content/format of your email

- IP reputation

- spam complaints from your database - regularly check how many people mark your emails as spam, not all providers send this data back to Marketo, but it is still important to monitor the overall trend because guess what happens when a large enough number of people report your emails as spam

I had to deal with a situation when our IP got into one of the biggest blacklists out there, we ended up getting a new dedicated IP and making major changes to our email marketing practices.

I wonder why some batch email campaigns have a normal deliverability (around 95%) and others, while being sent a couple days later, end up having around 80% of emails delivered, most of the rest blocked by SpamCop. Does it depend on which shared IP address is chosen today for our emailing (= good luck)? By the way, is a batch email campaign sent from one IP address or there are several IP addresses used for one campaign?

This is actually a common problem for Marketo (and to be fair most traditional ESP's).  Nearly all of Marketo's public IP addresses are listed at least once every day from the tests I ran for around a month.  Some of them are low risk RBL's such as NONSOLICITADO but in some cases you get listed on SPAMCOP. 

You unfortunately will suffer from this while you are on a public pooled IP.   I would investigate moving to a dedicated IP and investigate if you can get an MTA.

We're experiencing this with some of our strategic accounts.  Even though we have strong/active relationships with these accounts, their email servers are configured to block ALL mail received from Marketo (even with properly configured SPF/DKIM).  It's becoming more challenging than ever to make it to the user's inbox.  Which is why I advise all of our marketers that it's so important to use multi-tactic campaigns - not just email.

Not really. You can ask Marketo Support/deliverability to assist with unblacklisting the IP you are on.

What you CAN do is improve your list quality:

• if a lead hardbounces x2, set email invalid=T
• if a lead keeps soft bouncing (3 or more), either suspend it for 30 days or block it.
• blacklist spam traps (search threads and google for ideas)
• blacklist competitors
• consider using Single or Double Opt In globally to avoid sending unpermissioned emails.
• do not buy lists to import into system.