For example: If you are sending a lot of email messages to the same email domain like; hotmail.com, aol.com, Gmail.com or yahoo.com their SMTP mail  servers may  block your email domain or IP after a certain number of email messages--because they may have set their mail servers to block anyone that have sent more than 100 messages within a short period of time.

In other words, they can limit the volume of emails from any mail server by time and/or quantity as well as permanently barred (blacklist) an email domain or IP so that in future all all email from the IP or domain will be automatically deleted without notifying the sender.

Take note that the port 25 that your program uses to send email messages to recipient mailboxes and the port the program uses to establish connections with your email program is not the same thing.

You can use any port, including port 25 to establish a connection between your email program and your (desktop computer) local SMTP mail server program...
but
When your local SMTP mail server program actually deliver your email messages to the recipients mailboxes, it always use PORT 25 to transmit data to the remote mail server - because it is the default standard SMTP port. Therefore, if your ISPs have block it, your mails won't get through.

port 25 blocking a method used by ISPs like: Juno, Netzero, FunCow, AT&T, MSN, etc., are one of the many ways, to prevent spammers from using random ISPs to send unsolicited emails.

SMTP - Simple Mail Transport Protocol are used for sending and receiving email messages. It is a protocol governing email transmission and reception.

Email servers uses a variety of protocols to communicate with the Internet and SMTP, POP3 (Post Office Protocol) are for sending and retrieving mails from a mail server.

POP3 - Post Office Protocol Version 3 rfc1939 used for delivering messages to (client) mail readers (fetch mails from a remote mail server and store it locally).