When you import users you should have already obtained their consent to receiving e-mail communications from you. Otherwise, you will need to send them a request to confirm that they indeed wish to receive messages from you. This is required by consumer privacy regulations in most countries. Check with your local authorities to make sure you are aware of what the rules are in your country.
In the United States, for example, the CAN-SPAM Act of 2003 provides specific guidelines for "commercial emailers". You can find more information here:
Please refer to the MailUp Knowledge Base for extensive information about how to use MailUp in a way that complies with e-mail marketing "Best Practices" and consumer privacy regulations.
Compliance with the MailUp Anti-Spam Policy