If an attempt to deliver a message fails, the SMTP module can delay delivery of that message, or it can delay delivery of the entire host queue (if a connection with the host could not be established or an established connection was broken, etc). The Retrying panel allows you to control how the SMTP module makes attempts to deliver messages:
Retry Every: Use this setting to tell the SMTP module when it should retry to send a message if a connection fails. If you use a forwarding mail server, this option specifies when the module should retry to connect to that server if the previous connection failed for any reason. If you use the Directly to Recipients method, and a connection to some remote host (domain) fails, all messages directed to that domain will be suspended for the specified time. Usually SMTP systems suspend messages for 30 minutes.
for the first... then retry: Use these settings to tell the SMTP module when and how it should change the retry interval. Usually, you would tell the SMTP module to increase the retry interval to 1-2 hours after a message has spent more than 2 hours in the queue.
for: Use this setting to limit the number of attempts to deliver a message. If a message cannot be delivered within the specified period of time, the message is rejected and an error report saying that the host is unavailable is sent back to the message sender.
Messages with empty 'Return Paths': for: This setting specified the alternative Keep Trying value for messages with empty Return-Paths. These messages are used to report failed or successful delivery, and they can be discarded from the Queue after fewer attempts.
Delay Messages: When a receiving host returns a "temporary failure" (4xx) response for a message sending command (MAIL FROM, DATA, the "final dot"), this setting specifies when an attempt to send this message should be repeated.
Delay Recipient: When a receiving host returns a "temporary failure" (4xx) response for a message recipient command (RCPT TO), this setting specifies when an attempt to send the message to this recipient should be repeated.
Send Warnings After: Use these settings to tell the SMTP module when warning messages should be sent back to the message senders, notifying them about delivery delays.