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435 Unable to authenticate at present
If you receive the error "435 Unable to authenticate at present" while working with emails, may be in webmail or in the error log, then check the ownerships of the following directories and files which should be cpuser.mail.

/home/cpuser/etc
/home/cpuser/etc/domainname.com
/home/cpuser/etc/domainname.com/shadow
/home/cpuser/etc/domainname.com/passwd
If they are not set correctly (usually set to cpuser.cpuser for older versions of cPanel ) then give following command

/scripts/fixeverything
If the issue is for all the accounts, then please update exim with the command

/scripts/eximup --force
Before /scripts/eximup please verfiy if the /etc/exim.conf is not immutable (i.e. protected using attritbutes).