SMTP Error
                     How to Fix “SMTP Error: Could not 
Authenticate”?
SMTP or Simple Mail Transfer Protocol is an essential 
component of email communication that allows users 
to send and receive emails. SMTP authentication is a 
crucial security feature that ensures only authorized 
users can send emails through a server.
However, there may be instances when PHPMailer fails 
to work due to issues such as misconfigured SMTP 
settings or compatibility problems with the server. 
When this happens, it may trigger an SMTP error: could 
not authenticate the message and prevents users from 
sending emails.
What does “SMTP Error: Could not authenticate” mean?
Other common SMTP authentication errors include:
● SMTP Error: Authentication Failed
● SMTP Error: Username and Password not accepted
● SMTP Error: Authentication method not supported
What are the Causes of “SMTP Error: Could not authenticate”?
1. Incorrect Authentication Error: Incorrect login credentials are the 
most common cause of SMTP authentication errors. Users must ensure that 
they enter the correct username and password for their email account.
2. Authentication protocol not supported: Email clients and servers 
support different authentication protocols. If the email client does not 
support the authentication protocol used by the server, authentication will 
fail.
3. Incorrect SMTP port and SMTP host: Sometimes, users in 
a hurry, enter the wrong mail server details in the mail 
application. In addition to that a slight mistake in the hostname 
or inactive mail server can also result in an SMTP error: could 
not authenticate.
4. Misconfigured SMTP encryption settings: When it comes 
to secure email transmission, SMTP with encryption is the top 
choice for most users. The two encryption protocols that are 
commonly used for this purpose are SSL and TLS. However, it’s 
important to keep in mind that some mail servers may lack 
SSL/TLS support or have an expired SSL certificate, resulting in 
an SMTP error.
5. Password has expired: It’s not uncommon for servers to be set up to auto-
expire passwords, and sometimes app maintainers may forget to update them on 
time. This can lead to a situation where the mail server is unable to validate the 
password when the application tries to connect. Result in “SMTP error: could not 
authenticate”.
6. Firewall or antivirus software blocking authentication: A firewall or 
antivirus software can block authentication, preventing the email client from 
communicating with the server. Users can temporarily disable these programs to 
test if they are causing the issue.