1.17 Old *.mail files containing "XMRG FROM:" in ~listserv/spool seem to be slowing things down (similar to 1.16 but nothing in the log)

LISTSERV's spool directory has some old *.mail files in it that seem to be taking up processing time and are slowing things down. These *.mail files contain lines like

HELO LISTSERV.EXAMPLE.COM

XMRG FROM:<owner-TEST@LISTSERV.EXAMPLE.COM> VERSION=1

XDFN NAME="Jane User"

XDFN *FROM=owner-TEST*JANE**EXAMPLE*-COM@LISTSERV.EXAMPLE.COM

RCPT TO:JANE@EXAMPLE.COM

and sendmail appears to be having a problem with them. What do I do about this? Where are these files coming from?

These are mail-merge jobs. See the previous question for the solution to this problem.