XF 2.1 email notifications not working

[ZenFoo]

Particulars:
PHP version 7.3.9
MySQL version 5.7.27
XenForo: 2.1.4

Addons:
[SC] Online Status Indicator 2.0.0
[SC] Separate Sticky Threads 2.0.0
[TH] User Improvements 1.3.0 Patch Level 9
[XFA] Core 1.6.0
[XFA] Nodes Icon Tweak 3.0.4

Issue: no user receives email when options ticked to get them on watched content. Registration emails go through no problem, test emails as well. I'm using default transport method. I've check the server logs there are no errors, XenForo also reports no errors.

So what's my next step SMTP? I'd rather not do this.