3.20 Why does my sendmail process sometimes hang when connecting over a SLIP/PPP link?

Most likely, the problem isn't sendmail at all, but the low level network connection. It's important that the MTU (Maximum Transfer Unit) for the SLIP connection be set properly at both ends. If they disagree, large packets will be trashed and the connection will hang. For a more thorough discussion of this see question 4.8.