There are some checks performed when connections are made to our mail servers to send email. Triggering some of these limits could result in a temporary block, preventing or delaying the message being sent.
DoS detection, based upon your IP address:
25 SMTP connections within a 5 minute period results in a block for 60 minutes
User limits:
Default "recipients per message" limit: 50
Default "outgoing messages per hour": 100
How can I send legitimate mass email?
You will need to spread the sending of emails so that no more than 100 emails per hours go out, and no more than 25 SMTP connections per 5 minute period.
If using 'bulk mailer' software:
Edit the settings of your software so that the emails are scheduled to go at a rate of 1.65 per minute (or every 37 seconds); that's 99 per hour.
If using Microsoft Outlook:
Suggestion 1: Delay sending an individual message...
http://office.microsoft.com/en-us/outlook/HP052427901033.aspx
Suggestion 2: Delay ALL messages that you are about to send (rule can be turned on/off)...
http://office.microsoft.com/en-us/outlook/HA010549791033.aspx
Suggestion 3: Turn off the "Send Immediately When Connected" Option...
http://office.microsoft.com/en-us/outlook/HP012293011033.aspx
(Mail setup >>> Send/Receive)
Suggestions 2 & 3 will allow you to organize the email in the "Outbox" before it gets sent, perhaps creating a new sub-folder called "Queue"; this would allow you to select groups of email (say 20 at a time) and drag them from the "Queue" folder into the "Outbox" folder at appropriate intervals.