OS: CentOS 6.0
Bind Version: bind.i686 32:9.7.3-2.el6_1.P3.2 (from CentOS yum/cr repository)
Webmin Version: 1.570
In the bind module under Zone Defaults, there is a field "Default email address ". When you put a period on the end of the address, it is removed when a master type zone is created based on the settings in the zone defaults. This can be confirmed by viewing the webmin created zone file. According to the article at http://www.zytrax.com/books/dns/ch8/soa.html, a period should be used at the end of a fully qualified email address, otherwise the zone origin will be added to the end of it.
For example, if "Default email address " is set to "admin.mydomain.com.", in the created zone it will be added to the SOA record as "admin.mydomain.com" (note the missing period at the end). This will make the contact email address admin.mydomain.com.mydomain.com which is incorrect.
NOTE: It appears that it is valid to have a non-fully qualified email address (without the "." on the end) such that a contact email address of "admin" (no period on the end) would evaluate to admin.mydomain.com.
The current workaround I have found is to set the "Default email address " to "admin.mydomain.com.." (two periods on the end). One period is removed and the resulting contact address when creating a zone is "admin.mydomain.com." (one period).
When a zone is created based on the default zone settings, no period characters should be removed from the end of the "Default email address " value.