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: http://www.ftc.gov/bcp/conline/pubs/buspubs/canspam.shtmYour use of the MailUp service must at all times comply with the MailUp Terms & Conditions, including the MailUp Anti-SPAM policy. Remember that violation of the terms and conditions will result in immediate termination of your account, without any refunds.