Using the 1&1 mail server, you can send a message with no more than 100 recipients at one time and no more than 200 within five minutes. Please allow 10 seconds between sending e-mail messages. Packages less than 5 days old will be restricted to only 50 recipients within 6 hours. This restriction is in place primarily as a security precaution against SPAM, among other reasons.
When computing the settings for a mass mailing (where one e-mail is sent to a large number of recipients), it may be easier to consider how many messages can be sent per minute rather than every 5 minutes. 300 mails in 5 minutes equates to 60 emails every minute. To stay just below the threshold, send one mail every minute with no more than 55 recipients and this will remain under the mail server's limitation of no more than 200 emails within 5 minutes, no more than 100 recipients at once, and at least a ten second delay between mailings.
If each recipient is to receive a different message (rather than one e-mail with multiple recipients), then a 10 second delay (or more) is required between each e-mail transmission to adhere to the limits. This shrinks the possible mail transmission from 60 mails every minute to 6 mails every minute (assuming each e-mail sent is addressed to only one recipient).
If you are sending mails through a script (such as PHP), the emails will be sent using the 1&1 mail server by default. The above limits will still apply. If using a mail server configured on a 1&1 dedicated or VPS server, there is no restriction on e-mail transmission by 1&1.
Always remember when sending mass mailings to include an unsubscribe link for the recipients that allows removal from the mailing list if they do not wish to receive e-mail correspondence any longer. This also protects you against your mailings being flagged as SPAM messages.