When encountering the error "CodeIgniter SMTP mail not working," it indicates that the CodeIgniter framework's SMTP email functionality is failing to send emails through the configured SMTP server. CodeIgniter is a popular PHP framework that includes built-in support for sending emails via SMTP

This error appears because of diverse factors comprising:

1.  Malformed SMTP configurations causing unsuccessful connection establishment, e.g., erroneous email protocols, credentials, hostnames, or ports.
2.  Disabled SMTP authentication in email accounts hindering functionality, necessitating enabling options like Google's Less Secure Sign-In feature.
3.  Obstruction of SMTP connections resulting from restrictive firewalls blocking outgoing requests.
4.  Connection failures attributed to missing SSL or TLS specification depending on the SMTP providers' prerequisites.
5. Rare formatting anomalies involving single quote usage in frameworks like CodeIgniter leading to SMTP connection difficulties.

Solution - Here's How To Resolve It:

• Considerably disable both anti-virus guards and firewalls cautiously during testing phases.
•  Review SMTP providers' configuration details meticulously, adjusting elements such as SMTP hosts, ports, usernames, and passwords accordingly.
•  Apply mandated supplementary configurations whenever needed; for instance, activate Google's Less Secure Sign-In attribute.
•  Abide by each distinct SSL or TLS port stipulations set forth by individual SMTP providers.
•  Utilize double quotation marks instead of singles ones throughout SMTP configurations wherever feasible.