==== Provide required information ====
- iRedMail version and backend (LDAP/MySQL/PGSQL): iRedMail-0.8.1
- Linux/BSD distribution name and version: Centos 6
- Any related log? Log is helpful for troubleshooting.
====

Jun 26 14:29:25 master: Error: service(pop3): child 31855 killed with signal 6 (core dumped)
Jun 26 14:29:33 pop3-login: Info: Login: user=<backup@mydomain.com>, method=PLAIN, rip=172.16.1.13, lip=172.16.1.33, mpid=31859, TLS
Jun 26 14:29:33 pop3(backup@mydomain.com): Panic: file mail-index-sync-ext.c: line 209 (sync_ext_reorder): assertion failed: (offset < (uint16_t)-1)
Jun 26 14:29:33 pop3(backup@mydomain.com): Error: Raw backtrace: /usr/lib64/dovecot/libdovecot.so.0() [0x38e223da1a] -> /usr/lib64/dovecot/libdovecot.so.0() [0x38e223da66] -> /usr/lib64/dovecot/libdovecot.so.0() [0x38e221722a] -> /usr/lib64/dovecot/libdovecot-storage.so.0() [0x38e2699b23] -> /usr/lib64/dovecot/libdovecot-storage.so.0(mail_index_sync_ext_intro+0x1e6) [0x38e269a7d6] -> /usr/lib64/dovecot/libdovecot-storage.so.0(mail_index_sync_record+0x45f) [0x38e269c76f] -> /usr/lib64/dovecot/libdovecot-storage.so.0(mail_index_sync_map+0x234) [0x38e269d2d4] -> /usr/lib64/dovecot/libdovecot-storage.so.0(mail_index_map+0x86) [0x38e268ea46] -> /usr/lib64/dovecot/libdovecot-storage.so.0() [0x38e2689379] -> /usr/lib64/dovecot/libdovecot-storage.so.0(mail_index_open+0x210) [0x38e2689810] -> /usr/lib64/dovecot/libdovecot-storage.so.0(index_storage_mailbox_open+0xbc) [0x38e264ef7c] -> /usr/lib64/dovecot/libdovecot-storage.so.0() [0x38e266aa46] -> /usr/lib64/dovecot/libdovecot-storage.so.0() [0x38e266aad1] -> /usr/lib64/dovecot/libdovecot-storage.so.0() [0x38e26314f1] -> dovecot/pop3(client_create+0x1b4) [0x4049f4] -> dovecot/pop3() [0x403787] -> dovecot/pop3() [0x403de6] -> /usr/lib64/dovecot/libdovecot.so.0() [0x38e2234f3e] -> /usr/lib64/dovecot/libdovecot.so.0() [0x38e223569d] -> /usr/lib64/dovecot/libdovecot.so.0() [0x38e2235ff7] -> /usr/lib64/dovecot/libdovecot.so.0(io_loop_call_io+0x48) [0x38e2249148] -> /usr/lib64/dovecot/libdovecot.so.0(io_loop_handler_run+0x9f) [0x38e224a0cf] -> /usr/lib64/dovecot/libdovecot.so.0(io_loop_run+0x28) [0x38e22490d8] -> /usr/lib64/dovecot/libdovecot.so.0(master_service_run+0x13) [0x38e2236f63] -> dovecot/pop3(main+0x2e9) [0x403cd9] -> /lib64/libc.so.6(__libc_start_main+0xfd) [0x360d21ecdd] -> dovecot/pop3() [0x403649]

What actually causing this?
Clients started receiving error:

Sending of password did not succeed. Mail server <My server IP> responded: +

I had to restart my both primary and secondary servers to start operation smoothly as dovecot and postfix service restart did not resolve the issue. Moreover I had to delete .index files for the respective users in Maildir folders to stop errors at client side.