mailman not relaying email to external address

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http://serverfault.com – 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 = xxx.x ()