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<p>Quite possibly… it’s somewhat unprofessional, probably wouldn’t make or break your job application/research application/whatever, but you want to give a good, professional first impression for most of these things. However, your caltech username doesn’t have to be the one you use for resumes and everything. It’s a good idea to have at least one “formal” email address, but it doesn’t matter which one. I use my caltech email (which is of the form lastnamefirstinitial@caltech) for pretty much everything, but I know people who use a gmail account for formal purposes, and have an informal caltech email.</p>