Recently discussed on translator marketing forums has been the topic of buying an off-the-shelf mailing list to prospect for new agencies to work with. It is common practice, albeit a regulated one, in many industries to send an unsollicited snail-mail brochure or sales pack to prospects in certain areas or businesses. But things are different with email. The ease and immediacy of the medium is a large cause of the sheer virtual weight of emails that flood our inboxes every day. This might have reduced their efficiency if people didn't enjoy receiving messages intended for them; or at least those that they are happy to receive.
As soon as people become unhappy about a certain message's content they can flag it as spam. If enough people do so the sender can be blacklisted from the use of those email servers used. This can be a serious consequence for anyone trying to launch a marketing campaign. It is also something that is much harder to do for physical mail and perhaps why this format is still used today, despite most recipients considering unsollicted mail as 'junk'. Given that the prevailing public opinion is that messages are 'junk' or 'spam' unless requested/sollicited, the ethical position, and law, should stand on the side of the recipient; especially as they have little control over what is sent to them. And this in spite of the fact that the business deems the message's content to be useful to the recipient. That decision should (by law) be left to the recipient, usually done via the opt-in system, wherein the interested party formally acknowledges their request for more information from that source.
However the regulations and law see things slightly differently when it comes to business-to-business relationships, with prospecting emails generally being seen as acceptable provided they are within the scope of existing marketing mechanisms, that they feature an opt-out link, are not deceptive in headers, titles or contact details and that they are not sent to harvested email addresses. This particularly applies to the American CAN-SPAM legislation, although rulings are similar elsewhere. So it would appear not to be illegal to send unsollicited messages to companies (such as translation agencies) provided compliance with the above is maintained. But is it unethical? That is a question for the individual agency, with some finding it useful to receive applications from new translators, and others finding it more than an annoyance.
To avoid running the risk of annoying the best potential clients, it may then be prudent to phone to discuss your options and to arrange who to send an email to, if appropriate. This may be long-winded and impractical by comparison, but it might be good for your business if you care about the long-term contentment of your clients.