Oh, Boy. I am guessing you're running an old Solaris. Your sendmail is old. Nevertheless, I think you need to:
1) establish the machine that you are/will be using as your email 'gateway'. Is it a machine under your control, or is it at your ISP ?
2) 'equate' that machine's IP address (in your DNS or /etc/hosts) with the machine name 'mailhost' (and/or 'mailhost.domain.com'). If you are using /etc/hosts, then the line should look like this:
"10.4.5.6 mailhost.domain.com mailhost"
3) You could also 'grep' for the word 'mailhost' in your sendmail.cf file and replace the line that looks like
"DSmailhost.$w" with "DSISP-mailhost.ISP.com"
The first problem you are having is that while sending this email, your local sendmail is looking to connect to port 25 on the host 'mailhost' (this is a 'Sun-ism').
Since you have no host defined as 'mailhost' you cannot connect, and the mail fails.
So you either have to change the name of the host that sendmail is looking for (in the sendmail.cf file), or DEFINE a host named 'mailhost' for the local sendmail to
connect to (in DNS, or the /etc/hosts file).
The PROPER way to do this is to have an MX record in DNS that points to the "mail exchanger' that your domain is supposed to be using.
This will either be a 'Mail hub' that you maintain, or the 'Mail hub' that your ISP told you to use to send mail.
There may yet be other problems, but you cannot discover them until you correct this one.