You can probably install a second copy of the printer and set it's
properties to 300 dpi and then use that printer for Envelopes.
See the article "Changing the selected (current) printer in Word without
changing
the system default printer" at:
http://word.mvps.org/FAQs/MacrosVBA/ChangeCurPrinter.htm
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Hope this helps,
Doug Robbins - Word MVP