Good to know the test .php worked. I presume you left it set to simply use the PHP mail() function, and not specific SMTP ports or login info.
Does the "Send test email" in phpBB's own Email Settings section also work? And phpBB is presumably configured for "Use SMTP to send email: No"?
If the answer to everything above is "Yes", it sounds like maybe your mail provider (meaning your hosting service, since that's who's mail you're using) is dropping messages it believes to be spam. This would impact everyone who was supposed to receive that kind of email, because the mail is being dropped rather than forwarded by the hosting service.
Another possibility if "no mail succeeds, not even the phpBB test email" that your phpBB is configured to use your hosting service's mail servers, but you're providing a "sent by" or "from" email address which is not for a domain your hosting service handles, and looks "foreign" to them. (e.g. Using your hosting service's mail to try and send an email as "email@example.com
", or "firstname.lastname@example.org
", for example.) See Knowledge Base - EMAIL - Easy setup (NOT SMTP)
which talks about this, and recommends creating a mailbox through your hosting service, so that the mailbox will be from a domain your hosting service manages.
One other possibility is that only "some" users (yourself included) are failing to receive the mail, because their mailbox's inbound
mail system (not the phpBB host server's outbound mail system) has decided the message is spam. So some users would therefore receive the mail successfully, and others would not, depending on their inbound spam processing. In this kind of scenario where the outbound mail is actually being sent, you can use services like mail-tester.com to score how "spammy" your outbound mail appears to be, and things that could be changed to address that.
I've been in the scenario where my hosting service was dropping phpBB-standard emails because it triggered their spam filters, even though the "Send test email" in phpBB proved the ability to send an email was working. The workaround was to change the email templates (e.g. /language/en/mail/) such that they were more unique than what phpBB provided by default. So that the messages phpBB was sending wouldn't match the subject or content patterns the outbound spam filter was concerned with.
Ultimately I just chose to use an independent external mail service, and to setup phpBB to use SMTP to connect to that external service, rather than keep fighting the restrictive spam assertions. It's apparently not uncommon -- especially for lower-cost hosting tiers like mine, I imagine -- for the spam suppression to be aggressive, to push back on spammers utilizing those platforms.
For this same reason, note your hosting service may also block communication from your phpBB hosting service over standard SMTP ports like 465 or 25. So you might want to check whether external SMTP services are even reachable from your phpBB hosting service, if deciding to switch to using an external mail service.