They are email logging files under the Phone memory > logs > Eng > Ods. To disable logging state perform the following (for example on Nokia E72):
Select Menu > Email > Settings > Global Settings > Scroll down to 7 Logs printing > Disable.
There will still be an increasing number of the small log files. It is recommended to delete them periodically. You can safely delete all of log files by using File mgr. Go to Ods folder and select Options > Mark > Mark all > Options > Delete.
Note: Nokia E72 and E52 version 091.003 (and 091.004) does not generate the email log files by default.
Note: If the software upgrade is not available for your Nokia E72, E52 devices, you could install the following Email client version 3.25.0 which fix the logging issue. (Note: There is error on the version number when checking from Mail > Options > About).