The "local part" in the context of email addresses means the part to the left of the @ symbol. The warning is a bit of a surprise, since in general the owner of an email server is comparatively free to specify address format. There have been moves to allow other character sets to be used in domains so I'm a bit disappointed to see that, apparently, email doesn't support these in the local part yet Or it may be your SMTP server that generates this message. ASCII ("American Standard Code for Information Interchange") characters are, broadly speaking, the English alphabet, both upper and lower case, along with numerals, common punctuation marks and things like asterisks, slashes and brackets, but no accented characters. But even apparently simple things like spaces and dashes have cousins which at face value appear to be identical, but are not represented in the ASCII system. I'm thinking of non-breaking spaces, em-dash and en-dash in particular. But I don't know why your new computer should have provoked these warnings.