There are a couple of ways you can tell from my experience, but the "email suspended cause" value in your lead record is one of the best places to start.
The email suspended cause field will return a value if you get a soft bounce (spam block) response from your recipient. If you see an IP addresss of 199.15.215.XX or "em-sj-45.mktomail.com" (or a server address similar to this) in the suspended cause it means that your DKIM signing hasn't been set up or something within your DKIM isn't signing your emails.
Chris Willis - Fractional GTM Operations Consultant
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Anonymous
March 24, 2015
You can also use the filter Email Bounced Reason in a smart list and look for error codes there as well; just google any error codes that turn up for a description. Usually it's along the lines of too much server traffic or mailbox full type of thing
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Anonymous
March 25, 2015
I typically create a smart list which shows all the leads which had soft bounce for the email I am investigating. Like I do, I suggest creating a custom view to see these leads. Add 'email suspended', 'Email suspended cause', 'email suspended at', 'email bounce reason', email bounce date fields to that view. That way you can see all the relavent information right at one screen. When you drill down on a given lead, go to activity log and filter on 'Email' only. Thus you can only see the email send activities removing the clutter of all other change value etc activities. You can even create custom filter there if needed.