I apologize for any confusion, however this is the topic I meant this reply to go to. I am trying to determine where and how you are setting up your filters to get them to best work for your needed situation.
When adding a condition you should be presented with the following options (drop downs):
(#1) Include (or Exclude) | (#2) Membership: Field Selection(s) | (#3) Operator | (#4) Values from Drop Down Menu (#2)
The field, (#2) `membership:contact_email` (which produces the values for drop down (#4) is derived from the Email field in the Contact Information section in:
Membership > Users > edit a User > Info > Contact Information > Email:
If this field is left blank for all users within Membership then instead of getting a drop down menu for (#4) it would then produce a blank textbox. When there are no values to create a condition, adding a single character (or anything else) will not produce any results because there is not a value that can drive the condition.
The filter is not made to filter via the Email Marketing Email. It is made to filter based on Membership > Information sub-tab or Custom Directory information such as 'where company = "google"' or 'where number_of_children > 3' etc. If you have the same email set up for the Membership Contact Email as you do the Email Marketing email, you can send an email out to all users who have an email starting with 'B' However it will take a little bit different logic.
Because fields which contain values give you a dropdown you will have to do the following:
Include >= (First B email in the dropdown ) Exclude > (Last B email in the drop down)
NOTE: Once you have your conditions set you can then test your condition by clicking on the "Calculate Total" button:
This will produce the results of your current Query set. Please make sure that you calculate the total and view the included emails to verify that your includes and excludes are outputting the expected emails.