Unfrotunately, BCWipe is not intergrated in any email client.
However, there is a way to wipe the emails you no longer need.
To wipe a single email, the following should be done:
1) Delete the email.
2) Compact email database.
For example, to compact a folder in Thunderbird, right click that folder and select "Compact" from a popup menu.
3) Run Wipe Free Space on the disk where the database is located.