I have a setup of mailman with postfix on an ubuntu server 12.04. My problem is that mailing list emails are not forwarded to email addresses external to my institution. However

• the initial welcome email is received by everyone, internally and externally.
• in fact, a simple email from command line with mail is successfully sent to anyone
• after that, mailing list emails are only forwarded to internal addresses.
• the domain name I'm using for the server is not that of my institution who is hosting the server.

Here is my main.cf:

myorigin  = sub.myinstitution.tld
mynetworks = 127.0.0.0/8 xxx.xxx.xxx.xxx/16 # this is my institution ip range
relayhost = smtp.myinstitution.tld
inet_interfaces = loopback-only
local_transport = error:local delivery is disabled
virtual_alias_maps = hash:/etc/postfix/virtual
smtpd_recipient_restrictions = permit_mynetworks
myhostname = mywebsite.tld
mydestination = $myhostname, localhost.$mydomain, localhost

I also found these two links on serverfault and ubuntu forums, but neither of these solutions seem to do the trick for me.

Any help would be much appreciated.