I recently implemented vTiger CRM for a customer. One of the features they liked was the ability to attach e-mails from Outlook to a contact or company in vTiger. To test, we installed the vTiger Outlook plugin and put in the credentials of one user and it connected successfully. When we tried to attach an e-mail, however, it threw a 403 forbidden error:
After looking around online in the vTiger forums and on the blog, I found many others with the same issue, but no resolution (for almost 3 years). I thought about it and felt that maybe there was an issue on their server hosting vTiger. After all, you’d think vTiger would have put out an update or at least responded to comments. I looked into mod_security (on Apache servers) as this has been the culprit in other instances. Lo and behold, I found my customer’s IP in the log. I whitelisted their IP and vTiger Outlook started working.
So the solution is – whitelist the IP or mod_security rule.