Mail is being refused because a mailbox is full.
IF IT IS YOUR MAILBOX
cPanel > Email Accounts shows usage against quota. Clear space:
Empty Trash and Junk, which are usually the largest folders Sort by size in webmail and delete large attachments once saved elsewhere Archive old mail to your computer using Thunderbird or Outlook
Then raise the quota if the mailbox genuinely needs more, remembering it draws from your account's total disk allowance.
IF THE WHOLE ACCOUNT IS FULL
A hosting account at its disk limit stops delivering mail regardless of individual mailbox quotas. Check the cPanel statistics panel. Clean up files, old backups and cache folders as well as mail.
IF IT IS THE RECIPIENT'S MAILBOX
Nothing you can do at your end. Contact them another way and tell them.
PREVENTING IT
Set real quotas rather than unlimited, so one mailbox cannot consume the account Empty Trash and Junk regularly Archive annually Watch the cPanel disk usage figure, which includes mail
