The LMTP popup error message is an alert that indicates that all is not well with your local MTA server queue and needs attention.
LMTP protocal is an abbreviation for Local Mail Transfer Protocol. It is an alternative to SMTP - Simple Mail Transfer Protocol (SMTP) server. for situations where the receiving side does not have a mail queue acting on a forwarded email address in your cPanel hosted server account.
It demands for verification and allows for the fixing of any none existent emails used in the forwarded message. Proceed to verify that the forwarded email address was not created at some point in time and then deleted as a regular mailbox (POP3/IMAP) before. Check with your message queue if the the forwarded to emails are all Valid to allow for the queue to propagate appropriately. This also calls for a sound corporate email user policy especially with user data protection and regulation.
For troubleshooting, this is how to Fix cPanel LMTP error after RCPT to alert from your Microsoft Office outlook mail client.
Follow the path to file and
/home/mycpanelnusername/mail/broadnotes.com/userone
This will only apply where using forwarder only, and there is no mailbox for that user. Adjust accordingly if you have multiple /home-partitions (subdomain1->home, subdomain2->home etc.).
A mail delivery fact about an active LMTP protocol is that the local server can either accept an email message immediately or reject it hence cannot accept it at all. There is no attempt by the LMTP server to queue or redeliver a message that cannot be delivered immediately.
This task should be performed by an expert such as an administrator who manages your email and cPanel administrative duties. If not conducted by a qualified system admin, you risk losing copies of old messages in your inbox, sent items and copies of your junk and trash folders.
Backup your old file by keeping it in deleted items within your cPanel. Avoid permanently deleting the folder. Mitigate by holding it temporarily in trash. If the issue fails to resolve, restore the previous folder and permanently delete your new folder instead.
Check your account or email quota limit to ensure that you still have enough SSD storage space for your emails. If full, then the storage capacity is limiting new messages to flow to your inbox.
Ensure that you have not exhausted your quarterly hour limit issued by your hosting provider. This can potentially block you from sending out messages too.