You might be using various email services like Smartermail, Exim, MailEnable, etc. Now, you are facing a delay in email delivery to your Inboxes. Here, we have mentioned some reasons that can cause the delay in email delivery to your mailbox.
The following are the reasons that may cause delays in Inbox delivery. You can check all the below reasons within your email server to fix the late incoming issue to your mailbox.
(1) Greylisting is Enabled.
What is Greylisting?
Greylisting is a function used by mail servers to prevent incoming spam emails. Mail servers use the greylisting to find out if the sender server is legitimate or not. At first, greylisting will temporarily block any incoming emails from an unknown sender. A legitimate email server will attempt to deliver the email until it finally accepts or rejects by the recipient server. The recipient server will accept the legitimate email within some time. Emails sent for spamming purposes don't have such functionality to attempt a retry.
Now you should have understood why the emails are reaching late to your inbox due to greylisting. To overcome this issue, you must disable the greylisting.
(2) A Ton of Emails In the Email Spool/Queue.
Smartermail uses an email spool while Exim and MailEnable use email queue. Every email passes via email spool/queue to the recipient inbox. If the email size is large, it may be stuck in the spool/queue and cause the late incoming delivery. In another case, if an email server is under a spamming attack, there could be tons of emails in the spool/queue. You have to separate your legit emails and remove the spam emails from the spool/queue. Email spool/queue with a large number of emails can cause the late delivery to your legit emails.
(3) Insufficient Server Resources.
Insufficient server resources could be one of the reasons for late incoming email delivery. Make sure that your mail server doesn't have high RAM or Diskspace usage. Suppose an email server uses a high amount of RAM, so all the other processes, including email service, may be affected. It could be an issue with late email delivery. An email server with insufficient disk space can't save the incoming email file until there is any free disk. It will lead to late email delivery. You need to ensure that your email server has sufficient resources to run a smooth email operation.
(4) Antivirus Email Scanning Process.
An antivirus is useful to prevent virus attacks on your server. But if the scanning process includes real-time email scanning, it may affect email delivery. You can disable email scanning to avoid late email delivery.