# Email deliverability and testing tools: what AI models recommend, October 2026

GTM AI Recommendation Index, October 2026 Edition. Asked as "email deliverability tool" and as "inbox placement and email testing platform", six framings each, to fourteen models with search on, on behalf of a mid-market B2B company. Added to the October 2026 Edition on October 4, 2026; its answers are read by the distilled judge (ai-indexes-judge-qwen3-14b-run3), not the claude-opus-5 judge of the earlier categories. Page: https://gtm-ai-index.com/marketing/email-deliverability-and-testing/

**Standing:** GlockApps leads with 34% of first choices; verdict contested. 58 first choices across the direct, paraphrase, budget and scale prompts.

## First-choice share

| # | Product | Share | Negative rate | Labels |
|---|---|---|---|---|
| 1 | GlockApps | 34% | 7% | 57 |
| 2 | Google Postmaster Tools | 9% | 0% | 30 |
| 3 | MailReach | 7% | 7% | 30 |
| 4 | MXToolbox | 5% | 10% | 31 |
| 5 | Mail-Tester | 5% | 25% | 20 |
| 6 | Folderly | 3% | 5% | 20 |
| 7 | Microsoft SNDS | 3% | 0% | 15 |
| 8 | Validity Everest | 3% | 26% | 27 |
| 9 | Mailtrap | 2% | 8% | 26 |
| 10 | Email on Acid | 0% | 8% | 13 |
| 11 | Litmus | 0% | 23% | 22 |
| 12 | ZeroBounce | 0% | 25% | 20 |

## Each model's first choice on the direct prompt

- Claude Haiku 4.5: InboxArmy, MailBrace; alternatives Folderly, GlockApps, HubSpot, Mailsoftly
- GPT-5.4 mini: Mailgun Optimize; alternatives GlockApps
- Gemini 3.5 Flash: Allegrow, GlockApps; alternatives Mailgun Optimize, ZeroBounce
- Perplexity Sonar: GlockApps; alternatives Amplemarket, MailReach, NeverBounce
- Grok 4.1 Fast: GlockApps; alternatives MailReach, Validity Everest, Warmy.io
- Mistral Small: Folderly, GlockApps; alternatives MailReach, Zoho CRM
- DeepSeek V4 Flash: MailReach; alternatives GlockApps, Google Postmaster Tools, MXToolbox
- Llama 4 Maverick: GlockApps
- Qwen 3.7 Flash: GlockApps; alternatives 250ok, Instantly, Kickbox, Lemlist
- Kimi K2: GlockApps, Instantly; alternatives HubSpot Sales Hub, MailReach
- GLM 4.7 FlashX: GlockApps, MailReach; alternatives Folderly, Warmy
- MiniMax M2.5: Folderly, Mailtrap; alternatives EasyDMARC, MXToolbox
- GPT-6 Luna: Validity Everest; alternatives GlockApps
- Muse Glimmer 30B: GlockApps; alternatives MailReach, Validity Everest

## Sources the answers cite

79 of 84 answers came back with a source list, from 14 of 14 models. Sites named in the most answers:

- mailtrap.io: 35 answers, 41 citations
- saleshandy.com: 27 answers, 32 citations
- emailtooltester.com: 27 answers, 29 citations
- amplemarket.com: 27 answers, 27 citations
- instantly.ai: 25 answers, 37 citations
- guideflow.com: 24 answers, 29 citations
- sender.net: 23 answers, 29 citations
- mailreach.co: 21 answers, 34 citations

Pages named in the most answers:

- https://amplemarket.com/blog/best-email-deliverability-tools (26 answers)
- https://saleshandy.com/blog/email-deliverability-tools (23 answers)
- https://guideflow.com/blog/email-testing-software (20 answers)
- https://therankmasters.com/insights/email-marketing/best-email-deliverability-tools (20 answers)
- https://emailtooltester.com/en/blog/best-email-deliverability-tools (18 answers)
- https://omnisend.com/blog/email-deliverability-tools (18 answers)
- https://sender.net/blog/email-deliverability-tools (18 answers)
- https://brevo.com/blog/email-deliverability-tools (17 answers)
- https://instantly.ai/blog/email-deliverability-monitoring-tools (17 answers)
- https://myemailverifier.com/blog/email-deliverability-tools (17 answers)

## Warned against

- Mail-Tester: 5 of 20 labels negative. "Mail-Tester scores against a single seed address, so it can't tell you where your email lands across a multi-ISP seed network" (Claude Haiku 4.5, negative prompt)
- Litmus: 5 of 22 labels negative. "Litmus, for example, positions spam testing as part of QA and blocklist/authentication checks, not a guarantee." (GPT-5.4 mini, negative prompt)
- Validity Everest: 7 of 27 labels negative. "its pricing often starts at $20,000 to $65,000/year, making it massive overkill and too expensive for most mid-sized operations" (Gemini 3.5 Flash, paraphrase prompt)
- ZeroBounce: 5 of 20 labels negative. "ZeroBounce only handles the list side of deliverability, not the content or infrastructure side." (Claude Haiku 4.5, negative prompt)

## Record

- Method: https://gtm-ai-index.com/methodology/
- Raw judge labels and full responses: https://gtm-ai-index.com/data/
- License: CC BY 4.0. Cite as GTM AI Recommendation Index, October 2026 Edition, gtm-ai-index.com.