Sometimes IMAP server stops responding for new connections. Read & learn solutions to common IMAP problems.

Sometimes it may happen when IMAP/POP server stops responding for new connections.

If running cPanel/WHM, that issue may be accompanied with LFD alert as below:

Title: The service "imap" appears to be down.

Service Check Method: The system failed to connect to this service's TCP/IP port

Reason: TCP Transaction Log:
<< * OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE ACL ACL2=UNION STARTTLS] Courier-IMAP ready. Copyright 1998-2011 Double Precision, Inc. See COPYING for distribution information.
>> A001 LOGIN __cpanel__service__auth__imap__fQDNPHpJRXBFLELTHGmuDPuBlgBr_zPFrGekKQU7ewtlIzdCth8x1GdKiv4jUoI6 NWq3wQR7dj5vIwnVsX70v07_GvJhmYicHJMgED6go3fF2bHDGjlTVQWqojQtXEtS
<< * BYE Temporary problem, please try again later
imap: ** [* BYE Temporary problem, please try again later != A001 OK]
: Died

One of the solutions is to increase the amount of mail server threads (authentication daemons) running on the VPS.

To do that you need:

1. Login to WHM as root

2. Go to "Service Configuration" -> "Mailserver Configuration"

3. check the field "Number of Authentication Daemons"

4. Increase the number if required

5. Click "Save" (it will save new settings and restart mail server).