Spam emails are typically characterized by their unwanted nature, which often leads to a decrease in efficiency for users. These emails generally do not provide value and can clutter an inbox, making it difficult for users to find legitimate messages. This inefficiency can distract from productive work and consume unnecessary time as users sift through unwanted content.
While some spam emails can indeed be sent in bulk, not all spam must come from bulk senders; they can also be personalized or targeted to individuals. Moreover, while some spam can contain viruses, not every spam email is necessarily harmful in this way. Lastly, trusted senders are not the source of spam; rather, they often come from unknown or untrustworthy sources. Therefore, the defining characteristic highlighting the negative impact of spam on users is that they are unwanted and lead to decreased efficiency.