Not because they can, because they must . At least they must in most jurisdictions of most countries, except when the mail's subject is clearly labelled "PERSON

Not because they can, because they must . At least they must in most jurisdictions of most countries, except when the mail's subject is clearly labelled "PERSONAL" or there's some sort of automated classification in place. If someone receives an email coming from your employer's domain with a virus or a child porn video attached, your employer had better be able to identify the sender account through logs & audit trails.