diff --git a/Sources/NIOIMAPCore/Grammar/Command/Command.swift b/Sources/NIOIMAPCore/Grammar/Command/Command.swift
index 5df9ffb7e..b6272f17a 100644
--- a/Sources/NIOIMAPCore/Grammar/Command/Command.swift
+++ b/Sources/NIOIMAPCore/Grammar/Command/Command.swift
@@ -184,6 +184,12 @@ public enum Command: Hashable {
 
     /// Instructs the server to use the named compression mechanism.
     case compress(Capability.CompressionKind)
+
+    /// A custom command that’s not defined in any RFC.
+    ///
+    /// If `payload` contains multiple elements, no spaces or other separators will be output
+    /// between them. A `.verbatim` element must be used to output spaces if desired.
+    case custom(name: String, payloads: [CustomCommandPayload])
 }
 
 extension Command: CustomDebugStringConvertible {
@@ -297,6 +303,8 @@ extension CommandEncodeBuffer {
             return self.writeCommandKind_urlFetch(urls: urls)
         case .compress(let kind):
             return self.writeCommandKind_compress(kind: kind)
+        case .custom(name: let name, payloads: let payloads):
+            return self.writeCommandKind_custom(name: name, payloads: payloads)
         }
     }
 
@@ -456,6 +464,13 @@ extension CommandEncodeBuffer {
         self.buffer.writeString("COMPRESS \(kind.rawValue)")
     }
 
+    private mutating func writeCommandKind_custom(name: String, payloads: [Command.CustomCommandPayload]) -> Int {
+        self.buffer.writeString("\(name)") +
+            self.buffer.writeArray(payloads, prefix: " ", separator: "", parenthesis: false) { (payload, self) in
+                self.writeCustomCommandPayload(payload)
+            }
+    }
+
     private mutating func writeCommandKind_authenticate(mechanism: AuthenticationMechanism, initialResponse: InitialResponse?) -> Int {
         self.buffer.writeString("AUTHENTICATE ") +
             self.buffer.writeAuthenticationMechanism(mechanism) +
diff --git a/Sources/NIOIMAPCore/Grammar/Command/CustomCommandPayload.swift b/Sources/NIOIMAPCore/Grammar/Command/CustomCommandPayload.swift
new file mode 100644
index 000000000..ae0394dd0
--- /dev/null
+++ b/Sources/NIOIMAPCore/Grammar/Command/CustomCommandPayload.swift
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2020 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import struct NIO.ByteBuffer
+
+extension Command {
+    public enum CustomCommandPayload: Hashable {
+        ///  This will be encoded using `quoted` or `literal`.
+        case literal(ByteBuffer)
+        /// This will be encoded _verbatim_, i.e. directly copied to the output buffer without change.
+        case verbatim(ByteBuffer)
+    }
+}
+
+// MARK: -
+
+extension EncodeBuffer {
+    /// Writes a `CustomCommandPayload` to the buffer ready to be sent to the network.
+    /// - parameter stream: The `CustomCommandPayload` to write.
+    /// - returns: The number of bytes written.
+    @discardableResult public mutating func writeCustomCommandPayload(_ payload: Command.CustomCommandPayload) -> Int {
+        switch payload {
+        case .literal(let literal):
+            return self.writeIMAPString(literal)
+        case .verbatim(let verbatim):
+            return self.writeBytes(verbatim.readableBytesView)
+        }
+    }
+}
diff --git a/Sources/NIOIMAPCore/Pipelining.swift b/Sources/NIOIMAPCore/Pipelining.swift
index c4f229b0c..6392ead42 100644
--- a/Sources/NIOIMAPCore/Pipelining.swift
+++ b/Sources/NIOIMAPCore/Pipelining.swift
@@ -225,6 +225,14 @@ extension Command {
              .generateAuthorizedURL,
              .urlFetch:
             return []
+        case .custom:
+            return [
+                .noMailboxCommandsRunning,
+                .noUntaggedExpungeResponse,
+                .noUIDBasedCommandRunning,
+                .noFlagChanges(.all),
+                .noFlagReads(.all),
+            ]
         }
     }
 }
@@ -379,6 +387,8 @@ extension Command {
              .setMetadata:
             // TODO: Metadata dependencies?
             return [.mayTriggerUntaggedExpunge]
+        case .custom:
+            return [.barrier]
         }
     }
 }
diff --git a/Tests/NIOIMAPCoreTests/Grammar/CommandType/CommandType+Tests.swift b/Tests/NIOIMAPCoreTests/Grammar/CommandType/CommandType+Tests.swift
index d1a0e7f44..c5824045c 100644
--- a/Tests/NIOIMAPCoreTests/Grammar/CommandType/CommandType+Tests.swift
+++ b/Tests/NIOIMAPCoreTests/Grammar/CommandType/CommandType+Tests.swift
@@ -64,6 +64,17 @@ extension CommandType_Tests {
             (.create(.inbox, []), CommandEncodingOptions(), ["CREATE \"INBOX\""], #line),
             (.create(.inbox, [.attributes([.archive, .drafts, .flagged])]), CommandEncodingOptions(), ["CREATE \"INBOX\" (USE (\\Archive \\Drafts \\Flagged))"], #line),
             (.compress(.deflate), CommandEncodingOptions(), ["COMPRESS DEFLATE"], #line),
+
+            // Custom
+
+            (.custom(name: "FOOBAR", payloads: []), CommandEncodingOptions(), ["FOOBAR"], #line),
+            (.custom(name: "FOOBAR", payloads: [.verbatim(.init(string: "A B C"))]), CommandEncodingOptions(), ["FOOBAR A B C"], #line),
+            (.custom(name: "FOOBAR", payloads: [.verbatim(.init(string: "A")), .verbatim(.init(string: "B"))]), CommandEncodingOptions(), ["FOOBAR AB"], #line),
+            (.custom(name: "FOOBAR", payloads: [.literal(.init(string: "A"))]), CommandEncodingOptions(), [#"FOOBAR "A""#], #line),
+            (.custom(name: "FOOBAR", payloads: [.literal(.init(string: "A B C"))]), CommandEncodingOptions(), [#"FOOBAR "A B C""#], #line),
+            (.custom(name: "FOOBAR", payloads: [.literal(.init(string: "A")), .literal(.init(string: "B"))]), CommandEncodingOptions(), [#"FOOBAR "A""B""#], #line),
+            (.custom(name: "FOOBAR", payloads: [.literal(.init(string: "A")), .verbatim(.init(string: " ")), .literal(.init(string: "B"))]), CommandEncodingOptions(), [#"FOOBAR "A" "B""#], #line),
+            (.custom(name: "FOOBAR", payloads: [.literal(.init(string: "¶"))]), CommandEncodingOptions(), ["FOOBAR {2}\r\n", "¶"], #line),
         ]
 
         for (test, options, expectedStrings, line) in inputs {
diff --git a/Tests/NIOIMAPCoreTests/Parser/Grammar/GrammarParser+Commands+Tests.swift b/Tests/NIOIMAPCoreTests/Parser/Grammar/GrammarParser+Commands+Tests.swift
index ade0c62aa..5508945be 100644
--- a/Tests/NIOIMAPCoreTests/Parser/Grammar/GrammarParser+Commands+Tests.swift
+++ b/Tests/NIOIMAPCoreTests/Parser/Grammar/GrammarParser+Commands+Tests.swift
@@ -81,7 +81,7 @@ extension GrammarParser_Commands_Tests {
     }
 
     // Minimum 1 valid test for each command to ensure all commands are supported
-    // dedicated unit tests areprovided for each sub-parser
+    // dedicated unit tests are provided for each sub-parser
     func testParseCommand() {
         self.iterateTests(
             testFunction: GrammarParser().parseCommand,