First of all thanks to Digital Ocean community to help configuring lot of things! I have been trying to set up a mail server on my droplet which will work for multiple domains and multiple users with their mail client solutions, but mostly it didn't satisfy my requirements. I found some how-to-s in Digital Ocean articles but most of theme are for old releases of Ubuntu. If there is any about Ubuntu 16, please refer me, if not, please give me some links or instructions to do the job. Also I would like to inform that I use for some domain names Letsencrypt. Thank you in advance!