Don't forget to put in quotes around your command to be watched by the watch program.
it is this:
watch "lsof -i :25 | grep -v LISTEN | wc -l"
not this:
watch lsof -i :25 | grep -v LISTEN | wc -l
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watch "lsof -i :25 | grep -v LISTEN | wc -l"
watch lsof -i :25 | grep -v LISTEN | wc -l