ColdFusion 5: CFMAIL spool and undelivr Directories Limited to 65,535 Files
In ColdFusion 5 and earlier, mail files are generated using a hex naming convention from 0000.cfmail to FFFF.cfmail. As a result, neither the cf_root/mail/spool directory, nor the cf_root/mail/undelivr directory may contain more than 65,535 messages at any one time. If the upper limit is reached in either directory, an error is generated. For more information and a workaround, see the article below.
ColdFusion 5: CFMAIL spool and undelivr Directories Limited to 65,535 Files