Comcast blocks port 25
Comcast suddenly blocks port 25 without any notification. This may cause even compuer savvy people to waste a lot of time figuring out this after watching sending out email suddenly stopping to work, let alone laypersons.
If email client application such a Outlook, Thunderbird are used, the SMTP configuration in account settings needs to be changed to use an allowed port (e.g. 465). Email server such as SmarterMail, hMailServer also needs to be configured accordingly an alternate SMTP port. It is important to keep the existing port 25 for SMTP because most email servers use port 25, and each email server needs to communicate with others.
SMTP port of .Net applications using email also needs to be changedd accordingly.
(Originally written before 2018)