Yes, you can upgrade the service easily any time and the upgrade will take effect instantly. Upgrade will also enable your account right away if your account is temporarily disabled due to exceeding limit.
To upgrade, just login to your DNS Exit account, under "Mail Relay" section you should see the link "Upgrade" to upgrade the mail relay account.
Comment of calmelo nemhara:
I have been using yahoo for many years more than 15 years without any problem. I just got a problem now of failing to send mails . how can I get over this problem.
Added at: 2016-10-25 23:22