I have followed the steps below to enable IMAP and still things are not working.
sudo apt-get install php5-imap
sudo apt-get install php5-cli
sudo php5enmod imap
service php5-fpm restart
sudo service nginx restart
I can see the IMAP extension enabled under phpinfo and also can see the extension is loaded using command
dpkg -l | grep php5-imap
Server: Linux and using nginx
This should be a comment, but I haven't got the reps for comment, so...
Try to check your extension file, i.e. make sure that in /etc/php5/mods-enabled/imap.ini, the keyword extension=/path/to/imap.so is not commented (having ; or # as its beginning of line) and your /path/to/imap.so file exists.
Try to check if you had multiple php-fpm services running. Use systemctl list-unit-files | grep php to check. If you had multiple php-fpm services, make sure you had installed imap extension to the correct php-fpm instance.