Dial-up PC: sendmail doesn't send mails

["Alexander Farber (EED)" <eedalf_(_at_)_eed_(_dot_)_ericsson_(_dot_)_se>]

Hi,

I am unable to send any mails from my patched OpenBSD 2.9
computer using mutt or mail, here is what I get back:

     ----- The following addresses had permanent fatal errors -----
  eedalf_(_at_)_eed_(_dot_)_ericsson_(_dot_)_se
      (reason: 501 5.1.8 <eedalf_(_at_)_eed_(_dot_)_ericsson_(_dot_)_se>... Domain of 
      sender address alex_(_at_)_localhost_(_dot_)_my_(_dot_)_domain does not exist)

The IP-address and DNS servers for that computer are assigned 
dynamically via ADSL (ppp):

  pref:alex {102} cat /etc/myname
  pref

  pref:alex {103} cat /etc/resolv.conf 

  nameserver 212.185.249.84
  nameserver 194.25.2.129

  pref:alex {122} cat /etc/hosts
  ::1             localhost localhost.my.domain
  127.0.0.1       localhost pref
  #192.168.254.254        noname

  pref:alex {104} ifconfig tun0
  tun0: flags=11<UP,POINTOPOINT> mtu 1490
          inet 217.82.203.149 --> 217.5.98.52 netmask 0xffffff00 

  pref:alex {105} nslookup 217.82.203.149
  Server:  www-proxy.AC1.srv.t-online.de
  Address:  212.185.249.84

  Name:    pD952CB95.dip.t-dialin.net
  Address:  217.82.203.149

I suppose, that I should somehow tell my sendmail to use 
my provider's SMTP-server, which is mailto.t-online.de. 
I have also read in

  http://www.sigmasoft.com/~openbsd/archive/openbsd-misc/200105/msg00613.html

that I should probably add these lines to sendmail's m4-config file:

  define(`SMART_HOST',`smtp:mailto.t-online.de')
  MASQURADE_AS(`t-online.de')
  FEATURE(`masquerade_envelope')

But which file is it? /usr/src/gnu/usr.sbin/sendmail/cf/ostype/openbsd.m4 ?
And how do I compile and install it? Should I do:

  cd /usr/src/gnu/usr.sbin/sendmail && make clean && make obj && make build

or not? And where do I put these 3 lines, at the end of openbsd.m4?

Thanks in advance
Alex