### 6.3.3 Performance notes

One of the surest performance killers for Sendmail users is when Sendmail is configured to synchronously verify the recipient's host via DNS. If it does this for messages posted to it from Mailman, you will get horrible performance. Since Mailman usually connects via `localhost` (i.e. 127.0.0.1) to the SMTP port of Sendmail, you should be sure to configure Sendmail to not do DNS verification synchronously for localhost connections.