LMTP ( Local Mail Transfer Protocol ) in cPanel

The Local Mail Transfer Protocol (LMTP) is a derivative of Extended SMTP (ESMTP, sometimes referred to as Enhanced SMTP, is a definition of protocol extensions to the Simple Mail Transfer Protocol standard), the extension of the Simple Mail Transfer Protocol (SMTP). LMTP is an Application Layer protocol of the Internet Protocol Suite. It must not use the default unsecure port number (25) of SMTP, but it uses a TCP (Transmission Control Protocol) transport like SMTP. When the receiving side does not have a mail queue, such as a mail storage server acting as a MDA (Mail Delivery Agent), LMTP is designed as an alternative to normal SMTP for these situations.

Mail queues are an important requisite of SMTP. There are situations in which mail queues are not possible, LMTP is desirable, since a mail storage server should manage only its mail store and it will not allocate more storage for a mail queue. This is impossible with SMTP when there are multiple recipients for a mail message. SMTP can only point out the successful delivery or failure for all or none of the recipients and it also create a separate queue to handle the failed recipients. But in the case of LMTP, it can point out the success or failure to the client for each recipient and it also allow the client to handle the queueing instead. The client in this scenario would be an Internet-facing mail gateway.

LMTP issues in cPanel

LTMP is what Dovecot is now configured to use for local deliveries. We can discuss some LMTP issue on cPanel.

If you are getting email alerts from your server as

“The service ‘lmtp’ appears to be down” or alert for dovecot down. Then perform the following steps.

Try to restart the dovecot service first.

# restartsrv_dovecot

Then you must be getting error like:

mRT+MfXIqldQfgAAswxAZg: msgid=<[email protected]>: save failed to INBOX: Quota exceeded (mailbox for user is full)

Aug 10 10:56:56 server50 dovecot: lmtp([email protected]): Error: write(/home/user/mail/dovecot-quota.lock) failed: Disk quota exceeded

This error occurs because the cPanel user has been exceeded its quota. Dovecot is very sensitive so you will need to increase the disk quota for the user in order to resolve this issue. For this, please follow the below steps.

1)  Log in to WHM.

2)  Select the icon ‘Account Function’.

3) Click on the icon ‘Modify an Account function’.

4) Select the domain which you want to modify.

5) Increase the ‘Disk Quota’ and click on the ‘Save’ button.