The Unicode replacement character � (often a black diamond with a question mark in it) will often appear when imported data is invalid.
Here is a smart list you can setup and subscribe to be alerted of an issue with a list import.
In this example, I created the smart list in my master data management program.
Smart List - Unicode Replacement Character- List Import (Created in past 24 hours): Set the subscription to this list.
In this case I sent the date of activity for the past 24 hours but you can customize the time frame.
Now you can trouble shoot issues more quickly.
|Email Address||Company Name||First Name|
|firstname.lastname@example.org||German||ä ö ü|
|email@example.com||Spanish||á é í ó ú|
|firstname.lastname@example.org||Swedish/Finnish||Å å Ä ä Ö ö|
|email@example.com||Danish/Norwegian||Å å Æ æ Ø ø|
|firstname.lastname@example.org||French||À à Â â Ä ä È è Ô ô|
Here is a recent thread about Unicode issues in Marketo. There are other ways to import Unicode as stated in this thread.
Here is a wiki article if you are interested in learning more about the replacement character.