You may import a maximum of 10,000 addresses within a 24-hour period of time. This policy gives us a chance to review the quality of your list. For instance, if you upload a file with a high number of generic addresses or spamtraps, those addresses will need to be removed before you send. Otherwise, they can result in email blocking issues. You may also be asked to send a campaign to newly imported list members before importing another batch. This will allow us to monitor spam complaints, giving us further information about the quality of your list.
You can send the same campaign to each batch of addresses that you import, without the earlier batches getting a duplicate campaign, by targeting it based on signup date. Once your full list is imported, you can send a campaign to all list members at one time.