Alternate SMTP Ports – Send Email From Any Location
When sending outbound email from an email program (like Outlook or Thunderbird) or from a mobile device (like iPhone or Blackberry) that is not using Premium MobileSync, your program or device connects to our outbound email servers using an Internet protocol called “SMTP” (The Simple Mail Transport Protocol).
An email server, however, does lots of different things in addition to sending outbound email. It may allow checking of email via POP or IMAP, or checking your address book using LDAP, or other things. So, when your email program connects to the server it has to specify what it wants to do (i.e. send an email). It does this by connecting to a numbered “port” on the server. Port number “25” is the Internet standard for “regular outbound email”.
However, because port 25 is standard for outbound email, many ISPs, wifi networks, hotels, airports, and other locations that provide Internet access will arbitrarily block any connections to servers (except perhaps their own) on port 25 in order to stop spammers from using their services for the sending of spam, viruses, or malware and to prevent their IP addresses from being black listed.
So, if you try to configure your email program and can check your email fine but sending is failing, or if all was working well until you checked into some hotel, you are probably the victim of dreaded “port 25 blocking“. This can suddenly happen without notice even on your home ISP where everything has been sailing smoothly for years. We see it every day.
Avoiding Port 25 Blocking with Alternate Ports
Some service providers allow you to use alternate SMTP ports (e.g. ones other than 25) for sending email to get around blocks on using port 25 itself. For example, port 587 is a pretty standard port used for “submission of new email messages for outbound delivery”.
The problem with using another “standard port” like 587 is that its can also be blocked by many places that want to clamp down on outbound email traffic (like corporate firewalls and ISPs).
To resolve the port problem, LuxSci provides a number of standard and non-standard ports for outbound email sending. These ports have various properties and pros and cons. However, when our customers experience a problem with sending, changing to a different one of these ports usually solves the problem.
LuxSci’s Alternate SMTP Ports include:
| Port | Properties |
| 25 | Standard. Support Insecure SMTP and SMTP over TLS |
| 80 | Open on most firewalls* because this port is used for talking to web servers to get web pages. Supports insecure SMTP and SMTP over TLS |
| 465 | Standard port for SMTP over SSL. (How is SSL different from TLS?) |
| 587 | Standard port open on most firewalls. Supports insecure SMTP and SMTP over TLS. Also performs sender IP address masking and outbound email processing (e.g. automatic email encryption and other services). |
| 2025 | Nonstandard port open on most firewalls. Supports insecure SMTP and SMTP over TLS |
| 6025 | Nonstandard port open on most firewalls. Supports insecure SMTP and SMTP over TLS. Also performs sender IP address masking and outbound email processing (e.g. automatic email encryption and other services). |
| 6465 | Nonstandard port open on most firewalls. Supports SMTP over SSL. Also performs sender IP address masking and outbound email processing. |
*While port 80 is open on most firewalls, any that perform HTTP proxying and scanning will not permit SMTP traffic (i.e. email traffic) over that port.
Why So Many Ports?
We provide 6 different ports because:
- Some email clients can only connect securely on alternate ports via TLS or SSL. As you cannot support both TLS and SSL on the same port (why?), you need different alternate ports for each type of encryption.
- We like to offer standard and non-standard versions of the ports
All of these ports are available to all LuxSci SMTP clients, including those who use our High Volume outbound SMTP services.
