I would first check with Marketo Support and their Deliverability team.
Do you have a dedicated IP, or shared with other Marketo clients?
If it is not a dedicated IP, you run the risk of someone else on the cluster causing issues like this.
You can also check out services that will alert you as to red flags / blacklisting such as GetMeInTheInbox.com
It provides reports such as this one below:
IP Reputation
GMITI Score
199.###.###.###: Diamond
Sender Score:
199.###.###.###:
Score: 97
Risk: Low
Complaint Status: Low
Message Filtered: Low
Sender Rejected: Low
Unknown Users: Low
TrustedSource
199.###.###.###: Low threat level
SenderBase
199.###.###.###: Unknown
ReputationAuthority
199.###.###.###:
Reputation: Neutral
Clean: 54%
Viruses: 0%
Spam: 21.5%
Malformed Messages: 0%
Suspicious: 24.5%
Trustwave
199.###.###.###: Good
Cyren
199.###.###.###: No Risk
Sophos
199.###.###.###: Good Reputation
Webroot
199.###.###.###: Trustworthy
Mailspike
199.###.###.###: No Blocks Found
AOL
199.###.###.###: Good
IP Blocks
Office 365
199.###.###.###: Not Blocked
Cloudmark
199.###.###.###: Not Blocked
Symantec
199.###.###.###: Not Blocked
Trend Micro
199.###.###.###: Not Blocked
Barracuda
199.###.###.###: Not Blocked
AppRiver SpamLab
199.###.###.###: Not Blocked
Comcast
199.###.###.###: Not Blocked
Gmail
199.###.###.###: Not Blocked
Yahoo
199.###.###.###: Not Blocked
Road Runner
199.###.###.###: Not Blocked
EarthLink
199.###.###.###: Not Blocked
SurGATE
199.###.###.###: Not Blocked
Web.com
199.###.###.###: Not Blocked
Register.com
199.###.###.###: Not Blocked
Network Solutons
199.###.###.###: Not Blocked
Sonicwall
199.###.###.###: Not Blocked
Domain Reputation
TrustedSource
e.mydomain.com: Low threat level
e.mydomain.ca: Low threat level
promotions.mydomain.com: Low threat level
promotions.mydomain.ca: Low threat level
Webroot
e.mydomain.com: Trustworthy
e.mydomain.ca: Trustworthy
promotions.mydomain.com: Trustworthy
promotions.mydomain.ca: Trustworthy
Web of Trust
e.mydomain.com: Excellent
e.mydomain.ca: Excellent
promotions.mydomain.com: Excellent
promotions.mydomain.ca: Excellent
Domain Blocks
Barracuda
e.mydomain.com: Not Blocked
e.mydomain.ca: Not Blocked
promotions.mydomain.com: Not Blocked
promotions.mydomain.ca: Not Blocked
General Notes
199.###.###.###: This IP shows a high amount of inbox placement today as reported on ReputationAuthority and displays a positive email reputation on all sites. Over the past 30 days, this IP has hit 2597 spam traps.
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