Some email marketers may encounter IP banning when sending mass emails through 1and1Mail. Whether or not your IP will be blocked depends on the following things:
1. If you are using your company email accounts or paid email accounts to send mass mails, your email service provider will simply suspend your email account or SMTP connection. Your IP is safe in this case.
2. If you are using free email accounts from Gmail, Yahoo! Mail, or Hotmail, and using dynamic IP through dail-up. Your IP will be temporarily suspended, but it will be resumed soon, because the IP will be assigned to someone else next time.
3. If you are doing mass mailing through a static IP, it is very likely your IP will be suspended until you contact your ISP and explain the reason.
In 1and1Mail 3.2 (to be released at the end of Oct), the Auto IP Switcher feature can help avoid the above situations as much as possible and increase your email deliverability.