Today I had to enable mail on my server since Google Apps are not free anymore :/ and almost everything is running except I cannot receive mail from other domains. In Thunderbird I can check mails and if I send mail to myself then I'll receive it or if I send it to my gmail. But if I send it from my gmail account nothing happens. I don't receive any error on server, nothing.
Check the mx record of your domain if it points to sour server and that all records pointing to google apps are removed. It may take 24 hours until dns changes are propagated to all mirror dns servers.
