If your software follows the RFCs no delay is required. You should be able to send multiple emails in the same connection without delays.
If your software just sends messages and relies on delays rather than following the RFCs, there is no correct answer.
EDIT: If you read the RFCs you will see mail is sent using a conversational protocol. If your software follows that protocol, there is no need to insert delays between messages. Some software just uses delays where it should be following the conversation. Software that does that is very fragile and may break if the other end does not respond within a reasonable time.
Adding delays between messages when you send a bunch may or may not be a good thing. Delays give the server time to process the last message before a new message arrives. Not having a delay increases the change that multiple messages can be sent upstream with one connection. Unless you overload your mail server I would opt for no delay.