the punctuation mark to show the end of a sentence and is to be followed by a blank space before the first letter of the next sentence. Since an email address is NOT a sentence then it can not be a punctuation mark and not a period. It is a dot as in the use of a dot to indicate something in a list or similar type non-sentence type usage.
Oh, and as Eric reminds us, it's only in the USA that a sentence is finished with a period, most other countries call it a full stop.
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