diff --git a/stdlib/Markdown/src/Common/inline.jl b/stdlib/Markdown/src/Common/inline.jl
index d2855f27a7add..fd5134481e113 100644
--- a/stdlib/Markdown/src/Common/inline.jl
+++ b/stdlib/Markdown/src/Common/inline.jl
@@ -146,13 +146,10 @@ function _is_link(s::AbstractString)
 end
 
 # non-normative regex from the HTML5 spec
-const _email_regex = r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
+const _email_regex = r"^mailto\:[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
 
 function _is_mailto(s::AbstractString)
-    length(s) < 6 && return false
-    # slicing strings is a bit risky, but this equality check is safe
-    lowercase(s[1:6]) == "mailto:" || return false
-    return occursin(_email_regex, s[6:end])
+    return occursin(_email_regex, s)
 end
 
 # –––––––––––
diff --git a/stdlib/Markdown/test/runtests.jl b/stdlib/Markdown/test/runtests.jl
index 51f2e44c642cd..dfe80430a00d6 100644
--- a/stdlib/Markdown/test/runtests.jl
+++ b/stdlib/Markdown/test/runtests.jl
@@ -1251,3 +1251,12 @@ end
                                 """
     @test_throws ErrorException Markdown.latex(s2)
 end
+
+@testset "issue #42139: autolink" begin
+    # ok
+    @test md"<mailto:foo@bar.com>" |> html == """<p><a href="mailto:foo@bar.com">mailto:foo@bar.com</a></p>\n"""
+    # not ok
+    @test md"<mailto foo@bar.com>" |> html == """<p>&lt;mailto foo@bar.com&gt;</p>\n"""
+    # see issue #42139
+    @test md"<一轮红日初升>" |> html == """<p>&lt;一轮红日初升&gt;</p>\n"""
+end