What is the best alternative to Superhuman Mail on iPhone?

Spark Mail: AI Email Assistant, S tier, #1 in email writing.

Is there a free alternative to Superhuman Mail?

Yes: Spark Mail: AI Email Assistant (S tier, #1) is free to download.

Yes. 1 app ranks above it in email writing: Spark Mail: AI Email Assistant (#1).

Superhuman Mail is S tier, #2 of 6 in email writing. Here is everything that competes with it on iPhone, in tier order.

1
Spark Mail: AI Email AssistantS
Free download · ★ 4.6 · 82,695 ratings · score 96
Email app that combines multiple accounts into one inbox and surfaces priority messages first.
3
Canary Mail - AI EmailA
Free download · ★ 4.5 · 8,790 ratings · score 89
Email app that puts Gmail, Outlook, iCloud and other accounts into one inbox, with AI writing help, sorting, scheduling.
4
Friday AI Email WriterA
Free download · ★ 4.8 · 9,785 ratings · score 89
AI assistant that drafts emails and letters, proofreads text, and suggests writing edits from typed or voice input.
5
AI Writer: Email Letter EssayA
Free download · ★ 4.8 · 6,637 ratings · score 87
Free iOS app listed as Reference that generates writing for emails, letters and essays using AI, with voice dictation.
6
ParagraphAI: Writer & KeyboardB
Free + IAP · ★ 4.8 · 2,523 ratings · score 80
Free AI writing assistant and AI keyboard for drafting, editing, summarizing and translating text in 40+ languages.

What is the best alternative to Superhuman Mail on iPhone?: Spark Mail: AI Email Assistant, S tier, #1 in email writing.
Is there a free alternative to Superhuman Mail?: Yes: Spark Mail: AI Email Assistant (S tier, #1) is free to download.
Does anything rank above Superhuman Mail?: Yes. 1 app ranks above it in email writing: Spark Mail: AI Email Assistant (#1).