It depend what kind of domain. For TLD domains like .com , .org, .net, .biz and .info. You usually able to register up to 10 years maximum . While most of the country domain name usally only allow you to register up to 2 or 3 years only.
A section of our users use a different domain name for their email addresses. After rebooting the mail server this morning I noticed that they had all reverted back to the main domain name. I was wondering if this is normal? How can I set it so that they are forced to keep thier own domain name?
You can actually register as long as you liked, because you can renew it several times once you got it.
There are different rules for different registries for how much time you can register a domain name. .com, .net, .org, .info, .biz, .name, .travel, .us, .co.nz Domain name with these ext. can be registered for 1, 2, 5 or 10 ten years. If you are going to register a domain name for a longer period than discount rates will apply. .co.uk and .au Domain names with above ext. can be registered for 2 years at once.