In response to using Resend, this function is for sending a past campaign to any new contacts who've joined your contact list since you originally sent your email.
You can select either "Enter email addresses" (you are correct that this area limits the number of contacts you can add here to 25) or "New contacts since email was last sent." Thus we advise uploading a file of new contacts to that email list first and then selecting the latter option. Note: you can Resend an email campaign up to 85 days after the date you originally sent it.
Another option is to upload a list a new contacts to a new email list first, make a copy of your previously sent email campaign and schedule/send it to the new email list.
We're sorry for any confusion with how to add larger lists of contacts to your account. You'll want to upload your contacts from a file into a new email list before scheduling your email campaigns. (This allows more than 25 contacts to be added at one time.) Just make sure your file is formatted for Constant Contact! You'll click Contacts, Add Contacts and select Upload from file:
From here you'll follow the steps to add your contacts.
I would also encourage you check out our Sign-Up Form that you can customize for your website. Those who sign-up for your mailing list can be added directly into your account and linked with an Autoresponder Series. Autoresponders are designed to help you communicate with new contacts on specific email lists. An email series is triggered to start when a contact joins a list associated with it. Contacts can join these lists themselves using your sign-up form, or be added manually by you.