SortEmailAddresses←{
⍝ Sort email addresses ⍵ first by domain, then by mailbox
⍝ ⍺: Optional unique flag
     ⍺←0 ⍝: Default, return all addresses
⍝    ⍺←1  : Return unique email addresses (case-folded domains)
⍝ ⍵: Character vector with embedded new line characters
     admat←↑(⎕UCS 10 13)((~∊⍨)⊆⊢)⍵      ⍝ Split by new line
     dnorm←⎕C 2⊃↓⍉'@'(≠⊆⊢)⍤1↑admat      ⍝ Normalise domains
     dgroup←dnorm(⊂admat⍳⊢)⌸admat       ⍝ Indices grouped by domain
     dsort←admat∘{⍵[⍋⍺⌷⍤0 99⍨⍵]}¨dgroup ⍝ Indices sorted within each domain
     sea←admat⌷⍨⊂∊dsort[⍋∪dnorm]        ⍝ Sorted Email Addresses
     ⍺:sea⌿⍨≠⎕C@(∨\'@'∘=)sea            ⍝ Unique only
     sea
 }