Depends on what country you're in, but not illegal in US if you follow these tips:
Don’t use false or misleading header information. Your “From,” “To,” “Reply-To,” and routing information – including the originating domain name and email address – must be accurate and identify the person or business who initiated the message.
Don’t use deceptive subject lines. The subject line must accurately reflect the content of the message.
Identify the message as an ad. The law gives you a lot of leeway in how to do this, but you must disclose clearly and conspicuously that your message is an advertisement.
Tell recipients where you are located. Your message must include your valid physical postal address. This can be your current street address, a post office box you’ve registered with the U.S. Postal Service, or a private mailbox you’ve registered with a commercial mail receiving agency established under Postal Service regulations.
Monitor what others are doing on your behalf. The law makes clear that even if you hire another company to handle your email marketing, you can’t contract away your legal responsibility to comply with the law. Both the company whose product is promoted in the message and the company that actually sends the message may be held legally responsible.
