Customers who utilize Verizon FiOS internet service may find that they cannot send email via SMTP. This is because Verizon actively blocks outbound traffic on the default SMTP port 25. We recommend that customers who use Atlantech-hosted email solutions utilize the SMTP port 587 or SMTP port 465 to send their email. Please see this Verizon FiOS article regarding further details:

https://www.verizon.com/support/residential/internet/highspeed/general+support/top+questions/questionsone/124274.htm