diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index ae23c0acf..92290b385 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -2316,7 +2316,7 @@ def response_untagged
       end
 
       def response_tagged
-        tag = atom
+        tag = astring_chars
         match(T_SPACE)
         token = match(T_ATOM)
         name = token.value.upcase
@@ -3114,11 +3114,15 @@ def capability_response
         token = match(T_ATOM)
         name = token.value.upcase
         match(T_SPACE)
+        UntaggedResponse.new(name, capability_data, @str)
+      end
+
+      def capability_data
         data = []
         while true
           token = lookahead
           case token.symbol
-          when T_CRLF
+          when T_CRLF, T_RBRA
             break
           when T_SPACE
             shift_token
@@ -3126,7 +3130,7 @@ def capability_response
           end
           data.push(atom.upcase)
         end
-        return UntaggedResponse.new(name, data, @str)
+        data
       end
 
       def resp_text
@@ -3150,6 +3154,8 @@ def resp_text_code
         case name
         when /\A(?:ALERT|PARSE|READ-ONLY|READ-WRITE|TRYCREATE|NOMODSEQ)\z/n
           result = ResponseCode.new(name, nil)
+        when /\A(?:CAPABILITY)\z/ni
+          result = ResponseCode.new(name, capability_data)
         when /\A(?:PERMANENTFLAGS)\z/n
           match(T_SPACE)
           result = ResponseCode.new(name, flag_list)
@@ -3169,6 +3175,7 @@ def resp_text_code
           end
         end
         match(T_RBRA)
+        @pos += 1 if @str[@pos] == " "
         @lex_state = EXPR_RTEXT
         return result
       end
@@ -3269,7 +3276,7 @@ def astring
         if string_token?(token)
           return string
         else
-          return atom
+          return astring_chars
         end
       end
 
@@ -3299,34 +3306,38 @@ def case_insensitive_string
         return token.value.upcase
       end
 
-      def atom
-        result = String.new
-        while true
-          token = lookahead
-          if atom_token?(token)
-            result.concat(token.value)
-            shift_token
-          else
-            if result.empty?
-              parse_error("unexpected token %s", token.symbol)
-            else
-              return result
-            end
-          end
-        end
-      end
-
+      # atom            = 1*ATOM-CHAR
+      # ATOM-CHAR       = <any CHAR except atom-specials>
       ATOM_TOKENS = [
         T_ATOM,
         T_NUMBER,
         T_NIL,
         T_LBRA,
-        T_RBRA,
         T_PLUS
       ]
 
-      def atom_token?(token)
-        return ATOM_TOKENS.include?(token.symbol)
+      def atom
+        -combine_adjacent(*ATOM_TOKENS)
+      end
+
+      # ASTRING-CHAR    = ATOM-CHAR / resp-specials
+      # resp-specials   = "]"
+      ASTRING_CHARS_TOKENS = [*ATOM_TOKENS, T_RBRA]
+
+      def astring_chars
+        combine_adjacent(*ASTRING_CHARS_TOKENS)
+      end
+
+      def combine_adjacent(*tokens)
+        result = "".b
+        while token = accept(*tokens)
+          result << token.value
+        end
+        if result.empty?
+          parse_error('unexpected token %s (expected %s)',
+                      lookahead.symbol, args.join(" or "))
+        end
+        result
       end
 
       def number
@@ -3355,6 +3366,18 @@ def match(*args)
         return token
       end
 
+      # like match, but does not raise error on failure.
+      #
+      # returns and shifts token on successful match
+      # returns nil and leaves @token unshifted on no match
+      def accept(*args)
+        token = lookahead
+        if args.include?(token.symbol)
+          shift_token
+          token
+        end
+      end
+
       def lookahead
         unless @token
           @token = next_token
diff --git a/test/net/imap/test_imap_response_parser.rb b/test/net/imap/test_imap_response_parser.rb
index 4e470459c..4d5660ae8 100644
--- a/test/net/imap/test_imap_response_parser.rb
+++ b/test/net/imap/test_imap_response_parser.rb
@@ -234,6 +234,15 @@ def test_capability
     response = parser.parse("* CAPABILITY st11p00mm-iscream009 1Q49 XAPPLEPUSHSERVICE IMAP4 IMAP4rev1 SASL-IR AUTH=ATOKEN AUTH=PLAIN \r\n")
     assert_equal("CAPABILITY", response.name)
     assert_equal("AUTH=PLAIN", response.data.last)
+    response = parser.parse("* OK [CAPABILITY IMAP4rev1 SASL-IR 1234 NIL THIS+THAT + AUTH=PLAIN ID] IMAP4rev1 Hello\r\n")
+    assert_equal("OK", response.name)
+    assert_equal("IMAP4rev1 Hello", response.data.text)
+    code = response.data.code
+    assert_equal("CAPABILITY", code.name)
+    assert_equal(
+      ["IMAP4REV1", "SASL-IR", "1234", "NIL", "THIS+THAT", "+", "AUTH=PLAIN", "ID"],
+      code.data
+    )
   end
 
   def test_mixed_boundary