Mail-Tester occupies the entry-tier of deliverability tooling. The free version (3 tests/day) is what most operators encounter first — send an email to the unique address they provide, get a 0-10 spam score in 30 seconds. For a founder writing their first cold email sequence and asking 'will this actually land?', that 30-second feedback loop is genuinely valuable.
The paid tier ($12/mo for 50 tests/day) is the awkward middle. If you're testing 50 emails per day for deliverability, you're running enough volume that you need the per-provider breakdown GlockApps gives you — Gmail placement vs Outlook placement vs Yahoo placement, not just an aggregate spam score. Mail-Tester's design is single-test, single-score, which is the wrong shape for sender-reputation diagnostics at volume.
The useful frame: Mail-Tester is the deliverability stethoscope, GlockApps is the MRI machine. Both have legitimate use cases. A sender using only Mail-Tester at agency volume is under-tooled; a solo founder buying GlockApps at low volume is over-tooled. Match the tool to the operational stage.
On affiliate disclosure: Mail-Tester doesn't run a public affiliate program. /r/mail-tester routes to mail-tester.com unchanged. We cover it editorially because the search volume is real (founders searching 'free spam test' or 'why is my email going to spam') and we'd rather direct that traffic to a tool that fits the use case than not cover the category at the entry tier.