Laws and regulations protect against unwanted messages
An email address is personal. Most legal systems treat it that way, and regulate commercial emailing accordingly. The common target is spam — unsolicited bulk messaging — and the common remedy is to require that the recipient agreed in advance, can see who is writing, and can make it stop.
Emailing legislation, a means of protection against spam
The various national regimes converge on the same principle: prior consent before any commercial message. Collection should be opt-in, freely given and properly informed. Using addresses harvested without permission is prohibited, and can bring civil liability and, in several countries, criminal consequences. Every message must carry a clear way to unsubscribe, and the recipient keeps the right to see, correct and delete the personal data you hold about them.
One requirement that has disappeared is worth flagging, because it survives in a great deal of older guidance: there is no longer any obligation to declare your contact database to a regulator in the EU. The GDPR abolished the prior declaration regimes in 2018 and replaced them with accountability — you keep your own records of processing and must be able to demonstrate compliance on request.
Emailing legislation, the national particularities
The detail differs country by country, and the differences are not cosmetic.
In France, consumers must consent in advance, but prospecting a professional about matters relating to their job can proceed without prior consent provided they are informed and can object. In Belgium, consent is required under the Code of Economic Law, the exceptions are narrower than most senders assume, and an unsubscribe request must be acknowledged. In Switzerland, unsolicited commercial email is treated as unfair competition rather than a data protection matter, and can be prosecuted criminally. In Canada, CASL requires consent that the sender must be able to prove, along with a mailing address in every message and an unsubscribe honoured within ten business days.
If you send across borders, the practical rule is to design for the strictest regime your list touches rather than maintaining a different standard per country.
See also: emailing, definition of a marketing tool that has become a must.