There's not much we can do for a case that's simply stopped working and particularly involving php and its various "interesting" behaviors; what's usually involved with resolving these cases are the details of the error; the log messages, settings, any diagnostics, specific symptoms, etc. Details.
FWIW, there are always changes being made to a web server and its environment; I prefer to never assume the configuration is static. This whether the Apache logs — or something else on the system — is filling the available storage, random files are getting corrupted — as inherently happens with both soft and particularly hard errors that arise with computer storage devices — security attacks are ongoing and evolving, breached web servers are getting blacklisted for spam and their mail dropped, remote mail servers are updated and now detecting DNS configuring errors in the sending server and dropping messages, etc. All that — and more — happens... Even when "nothing changes."
In particular — though I'd expect your IT support would have investigated this — is there anything interesting or relevant found from reading your Apache web server logs? That's the usual first stop for these cases.
Are there any infestations of malware or the potential for a security breach here? You're using php and mail, so you're a target. See if the attackers made a mistake and corrupted some part of your configuration after establishing a breach.
Any particular patterns of the files involved when things go sideways, or any weird (non-execute) protections on files?
Any particular mail messages getting dropped? (If the mail is getting sent per your logs, have you checked the mail server logs on the receiving mail server to see if the messages are simply being dropped as spam?)
