This can make it difficult to create a straightforward new address for anyone without a highly unusual name or a good memory for complex email addresses.
In a bid to free up millions of new addresses, Yahoo will now provide webmail accounts under the new domains of ymail.com and rocketmail.com.
"We realised that we needed to expand the universe of Yahoo Mail," said John Kremer, vice president of Yahoo Mail.
"We have a lot of email addresses out there, and we want to make available more attractive ones."
Accounts will be activated in the same way as yahoo.com, offering country-specific extensions of the two new domains and unlimited storage capacity.
Existing Yahoo Mail account holders will also be given the option of migrating messages and contacts to accounts in the new domains.
Yahoo has reserved the most sought-after email addresses and will auction them off today, donating the proceeds to charity.
Recipients include The Breast Cancer Research Foundation, Ocean Conservancy, The Point Foundation, Right to Play and World Wildlife Fund.