Port 587 for SMTP
When you configure a mail client it's necessary to set the right SMTP port to make the email delivery process work correctly.
The default port is port 25, but being so extensively used it also conveys a lot of malware and spam: thus more and more ISPs are filtering the emails coming through it, especially if they are sent with a different mail server. And among them, there can be yours.
The solution? Switch to port 587.
Port 587 is another SMTP submission port that's supported by the vast majority of servers, and will definitely reduce the rate of rejected messages. That is particularly helpful if you're sending bulk or mass emails (e.g. newsletters): in this case you should also get a dedicated SMTP server to maximize your deliverability and handle unlimited messages.
So how to change your settings from port 25 to port 587? You simply need to open your client's configuration panel
and type "587" in the "Port" section, as highlighted in the screenshot above. If you are experiencing issues of email delivery, this will surely help: your SMTP server will start connecting through port 587 and take advantage of what is actually its preferred submission gateway.
But again, for the best result the you should also try a professional SMTP server that will zero the risk of being filtered out by your recipients' servers and ISPs. turboSMTP provides a free plan of 6.000 emails/month lifetime - you can start from here.