There are two possible issues which can affect sending email through clients such as Microsoft Outlook and Mozilla Thunderbird:
1) Your Local ISP (your cable company or DSL provider) doesn’t allow outside access to their mail (SMTP) server
Because of the huge problem with spam, most ISPs do not allow unauthorized access to their mail servers from outside their network. Many even require you to use their SMTP server for all outgoing mail. This prevents spammers from sending emails through these servers. The problem for the user arises when they travel outside of their home or office network. If you go to a coffee shop and log in, suddenly your mail server no longer recognizes you as inside your home network and blocks access. Logging in to SurfBouncer produces the same effect. When you log in to SurfBouncer, your IP address becomes that of our server on the internet. So, even though you are sitting at home, it looks to your ISP that you are trying to use their server from the outside and it blocks you.
Many ISPs also offer a secure way to log in to their servers in which you authenticate with your user name and password. In doing so, they allow you access from anywhere. Different ISPs handle this differently and the best way to find out how to configure your email client to use this alternate method is to call their tech support number. We have gathered some links below from some of the major ISPs which show how to configure your email client to use a different port and authentication to resolve this issue.
Yahoo Mac GoDaddy AOL Comcast Earthlink Gmail
2) You are trying to send outgoing (SMTP) mail through an un-authenticated connection on port 25.
Once again, due to problems with spammers and the recent increase in viruses that attach to unsuspecting users computers, this port is blocked. Spammers who once flooded open SMTP servers on the internet have reverted to using people’s computers, via viruses, to send spam email because these open servers have been closed. In order to prevent this spam from being sent through our servers, port 25 is blocked. The solutions described above will also solve this issue. Changing to an authenticated port for sending your email will allow all traffic to pass freely.
If your ISP does not provide for an authenticated outgoing mail server or you need a mail server that will work from anywhere in the world, we suggest you look into the service at the link below.
SMTP2Go Worldwide SMTP Service