It doesn't look like a problem with Mail.dll to be honest.
Those emails are received at completely different times (7:08, 7:11, 7:17, 7:23, 7:26, 7:32).
If those emails have multiple recipients (many To, Cc, Bcc addresses for a single email),
SendMessage may return SendMessageStatus.PartialSuccess, if some recipients are accepted but others aren't.
You should examine ISendMessageResult.RejectedRecipients and ISendMessageResult.ApprovedRecipients in such case.
You can also iterate over ISendMessageResult.RejectedRecipientsErrors to obtain actual errors for every recipient.
Mail.dll does not perform any kind of retries when sending messages.