Some ISPs set restrictions on the number of messages that you are allowed to send in a certain time period, eg 50 messages per hour, or the number of messages that you are allowed to send per connection.

If your ISP restricts the number of messages sent per connection, go to the VPOP3 Settings → Mail Connectors → Mail Senders → (sender name) → Settings tab. In the Max Messages Per Session option enter the number of messages you are allowed to send per connection.

When VPOP3 has sent the maximum number of messages, it will log off the ISP, and send the remaining messages later.

Note that on the Advanced tab you will probably want to make sure that Send smaller messages first and Reverse send order are turned off - this is so that the older messages are sent first, because VPOP3 may not be able to send all pending messages at once. Having either of these options turned on may mean that large, old messages are never sent, or have a long delay before sending.

If your ISP restricts the number of messages sent per time period, then you can use the same method as above, as well as restricting the number of connections that VPOP3 makes.

For example, if your ISP has a limit of 50 messages per hour, you can restrict VPOP3 to sending 10 messages per session, and also set VPOP3 to only connect 5 times per hour. This means it will send no more than 50 messages per hour.

To do this, calculate a suitable frequency of connection, and number of messages per connection to meet your ISP's restrictions. Then set the maximum number of messages as above.

Then, go to Mail Connectors → Connector Schedule in the VPOP3 settings. Edit your connection schedule so that the appropriate frequency of connection applies.

Also, make sure that If outgoing mail arrives whilst online, send immediately is turned off - otherwise that could cause more connections to take place than your ISP will allow.