well a quick "view source" on mine returned the following code:
<FORM ACTION="redirect.php" METHOD="POST">
<!--<CENTER> <IMG SRC="../images/sm_logo.png" ALT="Dreamhost SquirrelMail Logo"><BR>
<SMALL>SquirrelMail version 1.2.7<BR>
By the SquirrelMail Development Team<BR></SMALL>
</CENTER>
<CENTER>
<TABLE COLS="1" WIDTH="350">
<TR><TD ALIGN=CENTER BGCOLOR="#DCDCDC">
<B>Dreamhost SquirrelMail Login</B>
</TD></TR> <TR><TD BGCOLOR="#FFFFFF"><TABLE COLS=2 WIDTH="100%">
<TR>
<TD WIDTH="30%" ALIGN=right>Name:</TD>
<TD WIDTH="*" ALIGN=left>
<INPUT TYPE=TEXT NAME="login_username" VALUE="">
</TD>
</TR>
<TR>
<TD WIDTH="30%" ALIGN=right>Password:</TD>
<TD WIDTH="*" ALIGN=left>
<INPUT TYPE=PASSWORD NAME="secretkey">
<INPUT TYPE=HIDDEN NAME="js_autodetect_results" VALUE="0">
<INPUT TYPE=HIDDEN NAME="just_logged_in" value=1>
</TD>
</TR>
</TABLE></TD></TR>
<TR><TD>
<CENTER><INPUT TYPE=SUBMIT VALUE="Login"></CENTER>
</TD></TR>
</TABLE>
Now if you strip away all the formating and stuff, you get this:
<FORM ACTION="http://webmail.YOURDOMAIN.COM/src/redirect.php" METHOD="POST">
<INPUT TYPE=HIDDEN NAME="just_logged_in" value=1>
Name: <INPUT TYPE=TEXT NAME="login_username" VALUE="">
Password: <INPUT TYPE=PASSWORD NAME="secretkey">
<INPUT TYPE=SUBMIT VALUE="Login"></form>
make sure to change YOURDOMAIN and try it out. Not sure if it will work or not.