Sending as another address: choosing your port number and authentication type
If you would like to send mail as a different email address using the other domain’s SMTP servers, you’ll need to choose a port and authentication protocol that is supported by the other mail provider. In some cases, mail providers don’t support the recommended settings, in which case you may try other port/protocol combinations. If none of the secured options are available you may consider choosing the unsecured option.
Below you’ll find more information about the different authentication protocols:
What is the difference between SSL and TLS?
SSLSSL stands for Secure Sockets Layer. When using SMTP, this SSL mode typically refers to requiring a secured connection from the moment it connects the mail servers. Typically, SSL is associated with port 465, but this is not always the case. It’s best to check with your other mail provider for more details.
TLS (more formally known as STARTTLS)TLS stands for Transport Layer Security. When using SMTP, this type of authentication protocol begins with an unsecured connection to the server, followed by a STARTTLS command, then upgrades to a secured connection when actually transmitting data. Typically, TLS is associated with ports 25 and 587, but this is not always the case. It’s best to check with your other mail provider for more details.
Note that both are valid security protocols that ensure your data, such as your username, password, and messages, will be encrypted. This means that ordinary language is changed to code so no one can read what you’re sending. When you add another sender address, it is more important to know which port and authentication protocol your other mail provider supports.
What does it mean if my mail provider doesn’t support either SSL or TLS to connect to their mail servers?If your mail provider doesn’t support secured connections to their mail servers, we do offer port 25 in an unsecured connection. With an unsecured connection, your information won’t be encoded, which means it’s possible for someone to see your login credentials and message data when you connect.
Is it bad to use an unsecured connection?It’s not ideal, but every mail provider and ISP is different and will support different types of connections. If your mail provider or ISP doesn’t support a secured connection, you may want to consider using custom ‘From’ through Gmail’s servers.
Note that if you choose to send mail from an alternate address through Gmail’s servers, your Gmail address will still be included in your email header's sender field, to help prevent your mail from being marked as spam. Most email clients don't display the sender field, though some versions of Microsoft Outlook may display "From firstname.lastname@example.org on behalf of email@example.com."