At boot time sendmail looks for the FQDN (Fully Qualified Domain Name) of the local machine. If your machine is set up behind a firewall, proxy or NAT (Network Address Translator, such as Wingate and Sygate then DNS will not work or you will have an IP number, e.g. 192.168.0.7, used only at the LAN, but not registered in any DNS server.
The visible consequence of this is the boot process stopping at sendmail and waiting for a DNS timeout after what seems a long time. This mini-tutorial is a simplified version of RedHat article on the subject. All you have to do is to edit as root the file /etc/hosts and include a fictitious FQDN for your machine. For example, the machine GX300 with IP number 192.168.0.7, could have as the fictitious FQDN GX300.test.net, and be entered in the HOSTS file as follows:
Once you do this sendmail will read the FQDN from the HOSTS file and not stop or look for it in a DNS server, solving your problem.