Some ISPs set restrictions on the number of recipients that you are allowed to send in a certain time period, eg 50 messages per hour, or the number of recipients that you are allowed to send per message or per connection.
If your ISP restricts the number of recipients per message or connection, go to the VPOP3 Settings → Mail Connectors → Mail Senders → (sender name) → Settings tab. In the Max recipients per message or Max recipients per session options enter the number of recipients you are allowed to send per message or connection.
When VPOP3 has sent the message to the maximum number of recipients, it will log off the ISP, and send the message to the remaining recipients later. If you have set a limit of 20 recipients per message and you send a message to 100 recipients, then VPOP3 will send the message 5 times, each to 20 recipients.
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 recipients sent per time period, go to the VPOP3 Settings → Mail Connectors → Mail Senders → (sender name) → Settings tab. In the Max recipients: X per Y minutes boxes tell VPOP3 how many recipients your ISP allows per specified time period - eg
Max recipients: 50 per 60 minutes
If your ISP restricts the number of recipients 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 recipients per hour, you can restrict VPOP3 to sending 10 recipients per session, and also set VPOP3 to only connect 5 times per hour. This means it will send no more than 50 recipients per hour.
To do this, calculate a suitable frequency of connection, and number of recipients per connection to meet your ISP's restrictions. Then set the maximum number of recipients 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.