diff --git a/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift b/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift
new file mode 100644
index 0000000..58fb376
--- /dev/null
+++ b/Apps/MagnumOpus/ViewModels/ComposeViewModel.swift
@@ -0,0 +1,330 @@
+import SwiftUI
+import Models
+import MailStore
+import SyncEngine
+import SMTPClient
+
+enum ComposeMode: Sendable {
+	case new
+	case reply(to: MessageSummary)
+	case replyAll(to: MessageSummary)
+	case forward(of: MessageSummary)
+	case draft(DraftRecord)
+}
+
+@Observable
+@MainActor
+final class ComposeViewModel {
+	var to: String = ""
+	var cc: String = ""
+	var bcc: String = ""
+	var subject: String = ""
+	var bodyText: String = ""
+
+	var isSending: Bool = false
+	var errorMessage: String?
+
+	let mode: ComposeMode
+
+	private let accountConfig: AccountConfig
+	private let store: MailStore
+	private let actionQueue: ActionQueue
+
+	private var draftId: String?
+	private var savedTo: String = ""
+	private var savedCc: String = ""
+	private var savedBcc: String = ""
+	private var savedSubject: String = ""
+	private var savedBodyText: String = ""
+	private var autoSaveTask: Task<Void, Never>?
+
+	var isDirty: Bool {
+		to != savedTo || cc != savedCc || bcc != savedBcc
+			|| subject != savedSubject || bodyText != savedBodyText
+	}
+
+	init(mode: ComposeMode, accountConfig: AccountConfig, store: MailStore, actionQueue: ActionQueue) {
+		self.mode = mode
+		self.accountConfig = accountConfig
+		self.store = store
+		self.actionQueue = actionQueue
+
+		prefill(mode: mode)
+		snapshotSavedState()
+		startAutoSave()
+	}
+
+	deinit {
+		autoSaveTask?.cancel()
+	}
+
+	// MARK: - Send
+
+	func send() async throws {
+		isSending = true
+		errorMessage = nil
+
+		do {
+			let toAddresses = parseAddressList(to)
+			guard !toAddresses.isEmpty else {
+				throw ComposeError.noRecipients
+			}
+
+			let ccAddresses = parseAddressList(cc)
+			let bccAddresses = parseAddressList(bcc)
+
+			let domain = MessageFormatter.domainFromEmail(accountConfig.email)
+			let messageId = MessageFormatter.generateMessageId(domain: domain)
+			let from = EmailAddress(name: accountConfig.name, address: accountConfig.email)
+
+			var inReplyTo: String?
+			var references: String?
+
+			switch mode {
+			case .reply(let original), .replyAll(let original):
+				inReplyTo = original.messageId
+				if let origId = original.messageId {
+					references = "<\(origId)>"
+				}
+			case .forward(let original):
+				if let origId = original.messageId {
+					references = "<\(origId)>"
+				}
+			case .new, .draft:
+				break
+			}
+
+			let outgoing = OutgoingMessage(
+				from: from,
+				to: toAddresses,
+				cc: ccAddresses,
+				bcc: bccAddresses,
+				subject: subject,
+				bodyText: bodyText,
+				inReplyTo: inReplyTo,
+				references: references,
+				messageId: messageId
+			)
+
+			let formatted = MessageFormatter.format(outgoing)
+
+			let sendAction = PendingAction(
+				accountId: accountConfig.id,
+				actionType: .send,
+				payload: .send(message: outgoing)
+			)
+			let appendAction = PendingAction(
+				accountId: accountConfig.id,
+				actionType: .append,
+				payload: .append(mailbox: "Sent", messageData: formatted, flags: ["\\Seen"])
+			)
+
+			try await actionQueue.enqueue(sendAction)
+			try await actionQueue.enqueue(appendAction)
+
+			if let draftId {
+				try? store.deleteDraft(id: draftId)
+				self.draftId = nil
+			}
+
+			snapshotSavedState()
+			isSending = false
+		} catch {
+			isSending = false
+			errorMessage = error.localizedDescription
+			throw error
+		}
+	}
+
+	// MARK: - Draft Management
+
+	func saveDraft() {
+		let now = ISO8601DateFormatter().string(from: Date())
+		let id = draftId ?? UUID().uuidString
+
+		var inReplyTo: String?
+		var forwardOf: String?
+
+		switch mode {
+		case .reply(let msg), .replyAll(let msg):
+			inReplyTo = msg.messageId
+		case .forward(let msg):
+			forwardOf = msg.messageId
+		case .new, .draft:
+			break
+		}
+
+		let draft = DraftRecord(
+			id: id,
+			accountId: accountConfig.id,
+			inReplyTo: inReplyTo,
+			forwardOf: forwardOf,
+			toAddresses: encodeAddresses(to),
+			ccAddresses: encodeAddresses(cc),
+			bccAddresses: encodeAddresses(bcc),
+			subject: subject,
+			bodyText: bodyText,
+			createdAt: draftId == nil ? now : (try? store.draft(id: id))?.createdAt ?? now,
+			updatedAt: now
+		)
+
+		do {
+			if draftId != nil {
+				try store.updateDraft(draft)
+			} else {
+				try store.insertDraft(draft)
+				draftId = id
+			}
+			snapshotSavedState()
+		} catch {
+			errorMessage = error.localizedDescription
+		}
+	}
+
+	func deleteDraft() {
+		guard let draftId else { return }
+		try? store.deleteDraft(id: draftId)
+		self.draftId = nil
+	}
+
+	// MARK: - Prefill
+
+	private func prefill(mode: ComposeMode) {
+		switch mode {
+		case .new:
+			break
+
+		case .reply(let original):
+			if let sender = original.from {
+				to = MessageFormatter.formatAddress(sender)
+			}
+			subject = prefixSubject("Re:", original.subject ?? "")
+			bodyText = quoteBody(original)
+
+		case .replyAll(let original):
+			if let sender = original.from {
+				to = MessageFormatter.formatAddress(sender)
+			}
+			// CC = all other recipients minus self
+			let selfAddress = accountConfig.email.lowercased()
+			let others = (original.to + original.cc)
+				.filter { $0.address.lowercased() != selfAddress }
+				.filter { $0.address.lowercased() != original.from?.address.lowercased() }
+			cc = others.map(MessageFormatter.formatAddress).joined(separator: ", ")
+			subject = prefixSubject("Re:", original.subject ?? "")
+			bodyText = quoteBody(original)
+
+		case .forward(let original):
+			subject = prefixSubject("Fwd:", original.subject ?? "")
+			bodyText = forwardBody(original)
+
+		case .draft(let draft):
+			to = draft.toAddresses ?? ""
+			cc = draft.ccAddresses ?? ""
+			bcc = draft.bccAddresses ?? ""
+			subject = draft.subject ?? ""
+			bodyText = draft.bodyText ?? ""
+			draftId = draft.id
+		}
+	}
+
+	// MARK: - Auto-Save
+
+	private func startAutoSave() {
+		autoSaveTask = Task { [weak self] in
+			while !Task.isCancelled {
+				try? await Task.sleep(for: .seconds(10))
+				guard !Task.isCancelled else { break }
+				guard let self else { break }
+				if self.isDirty && !self.isSending {
+					self.saveDraft()
+				}
+			}
+		}
+	}
+
+	private func snapshotSavedState() {
+		savedTo = to
+		savedCc = cc
+		savedBcc = bcc
+		savedSubject = subject
+		savedBodyText = bodyText
+	}
+
+	// MARK: - Helpers
+
+	func prefixSubject(_ prefix: String, _ original: String) -> String {
+		// Strip existing Re:/Fwd: prefixes (case-insensitive)
+		var stripped = original
+		while let range = stripped.range(
+			of: #"^(Re|Fwd|Fw)\s*:\s*"#,
+			options: [.regularExpression, .caseInsensitive]
+		) {
+			stripped = String(stripped[range.upperBound...])
+		}
+		return "\(prefix) \(stripped)"
+	}
+
+	func quoteBody(_ msg: MessageSummary) -> String {
+		let dateFormatter = DateFormatter()
+		dateFormatter.dateStyle = .medium
+		dateFormatter.timeStyle = .short
+		let dateString = dateFormatter.string(from: msg.date)
+		let sender = msg.from?.displayName ?? "Unknown"
+		let originalBody = msg.bodyText ?? ""
+		let quoted = originalBody
+			.components(separatedBy: .newlines)
+			.map { "> \($0)" }
+			.joined(separator: "\n")
+		return "\n\nOn \(dateString), \(sender) wrote:\n\(quoted)"
+	}
+
+	func forwardBody(_ msg: MessageSummary) -> String {
+		let dateFormatter = DateFormatter()
+		dateFormatter.dateStyle = .medium
+		dateFormatter.timeStyle = .short
+		let dateString = dateFormatter.string(from: msg.date)
+		let from = msg.from.map(MessageFormatter.formatAddress) ?? "Unknown"
+		let toList = msg.to.map(MessageFormatter.formatAddress).joined(separator: ", ")
+		let subjectLine = msg.subject ?? "(No Subject)"
+		let body = msg.bodyText ?? ""
+
+		return """
+
+
+		---------- Forwarded message ----------
+		From: \(from)
+		Date: \(dateString)
+		Subject: \(subjectLine)
+		To: \(toList)
+
+		\(body)
+		"""
+	}
+
+	func parseAddressList(_ field: String) -> [EmailAddress] {
+		let trimmed = field.trimmingCharacters(in: .whitespacesAndNewlines)
+		guard !trimmed.isEmpty else { return [] }
+		return trimmed
+			.components(separatedBy: ",")
+			.map { $0.trimmingCharacters(in: .whitespaces) }
+			.filter { !$0.isEmpty }
+			.map(EmailAddress.parse)
+	}
+
+	private func encodeAddresses(_ field: String) -> String? {
+		let trimmed = field.trimmingCharacters(in: .whitespacesAndNewlines)
+		return trimmed.isEmpty ? nil : trimmed
+	}
+}
+
+enum ComposeError: LocalizedError {
+	case noRecipients
+
+	var errorDescription: String? {
+		switch self {
+		case .noRecipients:
+			return "At least one recipient is required."
+		}
+	}
+}
diff --git a/Apps/MagnumOpus/Views/ComposeView.swift b/Apps/MagnumOpus/Views/ComposeView.swift
new file mode 100644
index 0000000..6ce1a18
--- /dev/null
+++ b/Apps/MagnumOpus/Views/ComposeView.swift
@@ -0,0 +1,138 @@
+import SwiftUI
+import Models
+
+struct ComposeView: View {
+	@Bindable var viewModel: ComposeViewModel
+	@Environment(\.dismiss) private var dismiss
+
+	@State private var showBcc = false
+	@State private var showDiscardConfirmation = false
+
+	var body: some View {
+		#if os(macOS)
+		content
+			.frame(minWidth: 500, minHeight: 400)
+		#else
+		NavigationStack {
+			content
+		}
+		#endif
+	}
+
+	private var content: some View {
+		Form {
+			Section {
+				TextField("To", text: $viewModel.to)
+					#if os(iOS)
+					.textContentType(.emailAddress)
+					.keyboardType(.emailAddress)
+					.autocapitalization(.none)
+					#endif
+
+				TextField("CC", text: $viewModel.cc)
+					#if os(iOS)
+					.textContentType(.emailAddress)
+					.keyboardType(.emailAddress)
+					.autocapitalization(.none)
+					#endif
+
+				if showBcc {
+					TextField("BCC", text: $viewModel.bcc)
+						#if os(iOS)
+						.textContentType(.emailAddress)
+						.keyboardType(.emailAddress)
+						.autocapitalization(.none)
+						#endif
+				}
+
+				TextField("Subject", text: $viewModel.subject)
+			}
+
+			Section {
+				TextEditor(text: $viewModel.bodyText)
+					.frame(minHeight: 200)
+			}
+
+			if let error = viewModel.errorMessage {
+				Section {
+					Text(error)
+						.foregroundStyle(.red)
+				}
+			}
+		}
+		.formStyle(.grouped)
+		.navigationTitle(navigationTitle)
+		#if os(macOS)
+		.navigationSubtitle(viewModel.subject)
+		#endif
+		.toolbar {
+			ToolbarItem(placement: .cancellationAction) {
+				Button("Discard") {
+					if viewModel.isDirty {
+						showDiscardConfirmation = true
+					} else {
+						viewModel.deleteDraft()
+						dismiss()
+					}
+				}
+			}
+
+			ToolbarItem(placement: .automatic) {
+				Button {
+					showBcc.toggle()
+				} label: {
+					Label("BCC", systemImage: showBcc ? "eye.slash" : "eye")
+				}
+				.help("Toggle BCC field")
+			}
+
+			ToolbarItem(placement: .confirmationAction) {
+				Button {
+					Task {
+						try? await viewModel.send()
+						if viewModel.errorMessage == nil {
+							dismiss()
+						}
+					}
+				} label: {
+					if viewModel.isSending {
+						ProgressView()
+							.controlSize(.small)
+					} else {
+						Label("Send", systemImage: "paperplane")
+					}
+				}
+				.keyboardShortcut(.return, modifiers: .command)
+				.disabled(viewModel.to.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty || viewModel.isSending)
+			}
+		}
+		.confirmationDialog("Discard Draft?", isPresented: $showDiscardConfirmation) {
+			Button("Discard", role: .destructive) {
+				viewModel.deleteDraft()
+				dismiss()
+			}
+			Button("Save Draft") {
+				viewModel.saveDraft()
+				dismiss()
+			}
+			Button("Cancel", role: .cancel) {}
+		} message: {
+			Text("You have unsaved changes.")
+		}
+		.onDisappear {
+			if viewModel.isDirty && !viewModel.isSending {
+				viewModel.saveDraft()
+			}
+		}
+	}
+
+	private var navigationTitle: String {
+		switch viewModel.mode {
+		case .new: return "New Message"
+		case .reply: return "Reply"
+		case .replyAll: return "Reply All"
+		case .forward: return "Forward"
+		case .draft: return "Draft"
+		}
+	}
+}