Intro:
I had to change the server name of my mailman server. I changed it in /etc/mailman/mm_cfg.py as follows:
# Default domain for email addresses of newly created MLs
DEFAULT_EMAIL_HOST = 'mailman.myserver.com'
#-------------------------------------------------------------
# Default host for web interface of newly created MLs
DEFAULT_URL_HOST = 'mailman.myserver.com'

BUT! Some links in the mailman site were OK (new) and others were not OK(Old servername)

SOLUTION:
To remedy to this all the mailing list need to be internally modified to reflect the new hostmname in the sites URLs and the emails URLs.
Ref: https://mail.python.org/pipermail/mailman-users/2006-February/049052.html
Simply run the following 2 commands:
cd /usr/lib/mailman/bin/
./withlist -l -a -r fix_url -- -v

This runs withlist and tells it to lock the lists (-l) process all lists (-a) process by calling fix_url in the module fix_url.py with arguments of the list instance and -v which causes fix_url to report what it’s doing. The — is to separate the -v option for fix_url from the withlist options since there’s no listname to do that in this case.

For mailing lists with different URL then the site is suggesting the following:
———————————–
If you have more than one virtual host, you have to process the lists
one at a time with

bin/withlist -l -r fix_url listname -u url_host

but you could wrap that in a shell script to run the command repeatedly
for all the listname/url_host pairs.
———————————–