First off, great question!  

You did exactly what I would recommend as a first step - and normally, just scanning for a "bad" email (no "@" character, or no "." before the .com) will do the trick, but sometimes one will slip past you on first glance.  The way we do it here is by breaking the email list into groups, then after separating them by semi-colons, we post them into the email list field in small bunches.  Depending on how many emails in your full list, that could be anywhere from 20 at a time to 100 at a time.  Then we isolate the group containing the "bad" email and scan them individually looking for an invalid one.  

It sounds like a lot of work, but really we can normally knock it out in 5-10 minutes.  If you would be more comfortable, just send the list to adminhelp@bluesombrero.com (as an excel file ideally) with a note referencing this post and I will take a look at it for you and point out the email address causing the issues! 

Have a great day!

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Vice President of Operations - Blue Sombrero