Mailmanagr

Maybe. API access to Basecamp can only be turned on and off by the account owner. Assuming they’ve got it enabled though, you’re all set to go.

FREE or $7 per month, depending on what you need.

Messages travelling between your computer (or other device) and the Mailmanagr mail server are unencrypted. If you’ve got a Basecamp account that supports SSL, then communication between Mailmanagr and Basecamp is encrypted. We don’t store any of the information that gets sent through Mailmanagr; once it’s posted to Basecamp, it’s gone from our system.

You’ve more than likely got a copy of the message you sent in your “Sent Messages” folder. Sometimes e-mail providers will clear out these folders after a while to save space, but you should at least have a backup for a couple of weeks.
We back up all of our stuff, including the databases of addresses and such, but we can’t retrieve that file that you sent through Mailmanagr 3 days ago (see above).

We’ve tried not to ask for any information that we don’t need to operate (with the exception of your name on the signup form). Everything else is used to communicate with your Basecamp account and to allow data transfer between Mailmanagr and Basecamp.

We don’t store any of the information you send through Mailmanagr. Once a message has been successfully posted to Basecamp, it’s deleted from our systems along with any file attachments.

The free plan is limited to 20 addresses and can’t send file attachments. The paid plan has no restrictions, allows for “wide open addresses” (anyone can send e-mail to them), and will also be getting some other premium features that we’re working on.