The word mainstreams is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 18
Highest Scoring Words With Friends™ Play In The Letters mainstreams
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Seven Letter Word Alert: (59 words)
airmass, amentia, ammines, amnesia, amritas, animate, antsier, artisan, artsman, artsmen, atresia, inmates, inseams, inserts, instars, maimers, mantras, marinas, marines, marmite, martens, martian, martins, masters, miasmas, minaret, minters, mismate, misname, misseat, misters, nastier, raiment, remains, remints, restain, retains, retinas, satires, searats, seastar, seitans, seminar, sestina, simmers, sinters, sirname, smarten, smartie, smiters, stainer, stamens, stamina, stammer, strains, streams, tramman, tsarina, tsarism