emails in a new domainname

good day to all!

what would happen if you didn't change the domain name in your company email accounts, after not re-registering your previous domain name after it expired and used another registered name instead?
e.g.

new domain name = newdomainname.com
old domain name = olddomainname.com

monsky@olddomainname.com instead of monsky@newdomainname.com

i just added the newdomainname.com to the dns record without deleting the olddomainname.com. at present im not having a problem with the emails, what problems should i expect in the future?

tnx in advanced

From your post, I assume that you have control over MX records for yuor domains. If that is the case, emails to the new domain should be directed as specified by you, and emails to the old domain will continue until the domain registrar actually deletes the name server information which can be anytime from the actual expiry date onwards.

hi,
as long as you have setup your new domain and the associated email clients to fetch and send the mails to and from the mail servers i.e. pop and smtp servers, you will not have any problems.

if someone sends mails to your old domain address, as long as the entry for it remains in the mx records the mails will not bounce back with error codes. but the day the mx records are removed for your olddomain.com, from that day the mail sent to that domain, will be responded by the smtp server on the sender's end as "domain not found or email address not found"

emails to the old domain will continue until the domain registrar actually deletes the name server information which can be anytime from the actual expiry date onwards.

yes you are both right, i was obliged to use the new domain name because few days ago we cant receive email anymore.