This error is due to an invalid characters within the specified Email addresses within The Raiser's Edge.
To locate the problem record do the following:
Create a Constituent export and include the same query
Export the Constituent ID and the Email address type being used in Online Express.  If there are more than one Email type, select multiple Email addresses and types.
Export the file to Excel
Utilize formulas and filters within Excel that help identify invalid email addresses.

All email addresses must have an "@" a "." and cannot contain any spaces.  Although there are potentially other invalid characters, these are the most common issues.
This is used for illustration purposes for the types of formulas needed.

Constituent ID                                    Email Address                                                           "@" formula                                                      
12345                                                                                 =IF(COUNTIF(B2,"*@*"),"","invalid email")
                                                                                                                                            "." formula
                                                                                                                                  =IF(COUNTIF(B2,"*.*"),"","invalid email")
                                                                                                                                            No spaces formula
                                                                                                                                  =IF(COUNTIF(B2,"* *"),"invalid email","")

Working with formulas and filters in Excel is outside the scope of Blackbaud Product Support.  Please contact the appropriate software vendor or IT professional for assistance with this process.