The Mails from Joomla to customer as well as vendor are being marked as "phishing" mails. As per solutions provided in the other posts, I checked the header and it is being marked as "Spf-Fail". What is the solution to this issue?
Sender Policy Framework (SPF) is is an email validation system designed to prevent email spam by detecting email spoofing, a common vulnerability, by verifying sender IP addresses.
You can probably fix this by changing the sender and perhaps the reply-to email addresses to match the originating domain name.
You need to: