[wellylug] can't send mail - postfix etc

[veganforlife]

Hi all

Once again after moving to 6.06 I find some of my mail is
not being delivered.

(What I can't explain and don't understand is that when I
first had 6.06 it was all working
fine and now I am using my back up of 6.06 on my other
removable drive as I installed
7.04 over the top of the first 6.06, and now find these
problems on my backup hdd)

I am unable to send to the wellylug (webmail now) and also
paradise and
about half my mail is not arriving.

In my etc/postfix/main.cf  myhostname is shown as localhost

In my /etc/hostname it is shown as lyndsay-desktop
I tried changing that to localhost and rebooting but none of
my test messages have arrived at wellylug.

Also I tried to install sendmail, with apt-get install and
it generated some error
messages including that mutt wouldn't work without postfix,
and after the install
mutt was not there so I have gone back to postfix.

I am groping in the dark can anyone give me suggestions as
to where to look
next and what might be wrong. I have had previous issues
when I got Breezy last
year with mail being rejected and someone told me it was
because my hostname was wrong.
The problem is what should it be and in which places do I
need to change it?

Thanks for anything on this,
regards,
Lyndsay