Guidelines for establishing SMTP connections to Yahoo Mail
To facilitate the smooth flow of messages between legitimate senders and Yahoo Mail, we offer the following recommendations:
Limit the messages you send per connection
Yahoo Mail accepts a maximum of 20 messages per SMTP connection. We encourage you to cap the number of messages you send to Yahoo Mail to fall within this per-connection limit.
When this limit is reached, no further messages will be accepted for delivery as our server automatically terminates the connection (without giving an error code). If you are sending messages to a significant number of Yahoo Mail users, the suggestions below will help ensure uninterrupted delivery for your messages.
Reestablish connections if you do not get an error code
When our server terminates your connection after 20 messages have been accepted, you may try to reconnect to our MX servers immediately thereafter.
Open concurrent connections
You may open concurrent connections from the same server to facilitate efficient transmission of your messages. However, while we do not publish specific guidelines for the numbers of connections you can concurrently use, we ask that you treat our resources with respect. The more you take, the fewer there are for others, which may force us to de-prioritize connections from your server(s).