LOGFILE is an optional string specifying the name of a log file to which EMAILX will output (possibly) useful tracing and/or debugging information. The amount, or degree of detail, is determined by the next parameter (loglvl). Beware that if you send a lot of mail, and especially if you opt for a high degree of detail, this file can rapidly grow quite large. The expectation is that installations might temporarily use it for debugging problems with the SMTP server (for which it is extremely useful) and otherwise turn the feature off or set it to errors only. As with all of the other parameters which specify filenames, you can use AMOS or native syntax here.
