Are you on a Marketo Shared IP address or a dedicated one? If you are on a shared address then it is pretty much a guarantee that one of these in a day will be marked on one or more blacklists (RBL) - it happens ALOT.
We noticed exactly the same issue and I dug deep into this and this is what I found/know:
If your instance sends via a server which has been blacklisted (for recipient servers who respect and use this mechanism) it will send the emails it can and any emails which do are blocked are marked as Email Suspended = True. A blacklist block is actually a softbounce but Marketo marks and sees it as a hard bounce this is owing to the fact the it is in the system as a hardbounce code "Category 1 – spam block" (Email Suspended=true (for 24 hours)).
The Email Suspended flag that I can see does not get reset after 24 hours - and I am trying to work out if its a bug or by design (I assume the former or I am missing some data to check / know) Update: Marketo will never update this status - it is a product defect/fact of life. The engineering required to change this status is too great.
If an IP address is listed on one day, the next campaign will send from a non-listed IP where possible. Note: the where possible - it is possible that your next server selection is actually also blacklisted and you need to wait until the selection process chooses a server which is not blacklisted.
RBL - delisting is done automatically by (some) anti –spam providers and normally completed between 24 and 48 hours. During the phase of delisting, the IP address will not be used in any mail campaign by those instances hosted on the POD that utilises the range.
The following IP addresess are, from what I can see are the worst offenders:
199.15.214.200 (em-sj-00.mktomail.com)
199.15.214.199 (em-sj-99.mktomail.com)
>> Does Marketo run tests on their shared servers to ensure companies like us aren't negatively impacted by other customers...
Not that I know of no.
As of today thsi is what I know of the IP ranges in use in San Jose and London
| IP | Blacklisted | # of times | Region | RBLs |
| 199.15.215.77 | Yes | 5 | NA (San Jose) | DRMX, NoSolicitado, OSPAM, SPAMCANNIBAL, UCEPROTECTL1 |
| 199.15.215.78 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.215.79 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.215.80 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.215.81 | Yes | 4 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL, UCEPROTECTL1 |
| 199.15.215.82 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.214.202 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.214.201 | Yes | 3 | NA (San Jose) | DRMX, NoSolicitado, SPAMCANNIBAL |
| 199.15.214.200 | Yes | 2 | NA (San Jose) | DRMX, NoSolicitado |
| 199.15.214.199 | Yes | 2 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL |
| 199.15.214.45 | Yes | 2 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL |
| 199.15.214.46 | Yes | 4 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL, ZapBL |
| 199.15.214.47 | Yes | 4 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL, ZapBL |
| 199.15.214.48 | Yes | 1 | NA (San Jose) | NoSolicitado |
| 199.15.214.49 | Yes | 3 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL, UCEPROTECTL1 |
| 199.15.214.50 | Yes | 3 | NA (San Jose) | NoSolicitado, SPAMCANNIBAL, UCEPROTECTL1 |
| 185.28.196.15 | Yes | 6 | EMEA (London) | CASA CBL, CASA CBLESS, CASA CBLPLUS, NoSolicitado, SORBS SPAM, UCEPROTECTL1 |
| 94.236.119.58 | Yes | 5 | EMEA (London) | CASA CBL, CASA CBLESS, CASA CBLPLUS, NoSolicitado, SORBS SPAM |
| 38.188.97.188 | No | 0 | EMEA (London) (US based) |
| 94.236.119.5 | Yes | 3 | EMEA (London) | CASA CBL, CASA CBLESS, CASA CBLPLUS |
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