Learn Hardcore Japanese: sankatouroku no meeru ga mada todokanai noni, simekiri ha kyou desu. - The registration email hasn’t arrived yet, but the deadline is today.

届く (とどく) is the default intransitive for things reaching you (mail, parcels, email): メールが届く.
来る can be used casually for messages: メールが来る (more conversational).
受け取る is transitive “to receive (and take possession of),” used when you actively receive something: 荷物を受け取る. For email status, 届く/来る is more natural.
Related: 届ける is the transitive “to deliver.”

Why is it メールが and not メールは?

が marks the subject/new information: the email (subject) hasn’t arrived yet. は topicalizes and can imply contrast or shared topic. メールはまだ届かないのに… can sound like “As for the email (unlike other things), it hasn’t arrived…,” adding a contrastive feel. が is the neutral choice here.

What does 参加登録のメール mean, and could it be 参加登録メール?

参加登録のメール = the email about participant registration. The の links a noun modifier to a head noun. 参加登録メール (a compound) is also fine and a bit more label-like/technical. With の, it can feel a touch more explanatory.

Is まだ with a negative the right way to say “not yet”?

Yes. まだ + negative = not yet.

まだ届かない = It hasn’t arrived yet.
Contrast: もう届いた = It already arrived; もう届かない = It will no longer arrive (too late/impossible).

Difference between 届かない and 届いていない?

届かない: simple negative (doesn’t/won’t arrive). With まだ, it commonly means “hasn’t arrived yet.”
届いていない: not in the state of having arrived yet (progress/result focus). In everyday speech with まだ, both are natural: まだ届かない ≈ まだ届いていない. The -ている form can feel a bit more descriptive.

Is it okay to mix plain and polite forms like 届かないのに、…です?

Yes. Subordinate clauses before のに/けど/から/ので usually use the plain form. The main clause can end in polite です/ます. If you want full politeness, you can rephrase:

まだ届いていませんが、締め切りは今日です。 Using ます directly before のに (e.g., 届きませんのに) is generally avoided.

Why 締め切りは今日です and not 今日が締め切りです or 今日は締め切りです?

All are grammatical; they differ in focus.

締め切りは今日です: topic = deadline; focus = it’s today (what day is the deadline? Today).
今日が締め切りです: focus on “today” as the identifier (which day is it? It’s the deadline).
今日は締め切りです: topic = today; says that today happens to be the deadline. Choose based on what you’re emphasizing.

How can I make the whole sentence more polite/businesslike?

Use humble/polite forms and business vocabulary:

参加登録のメールがまだ届いておりませんが、締め切りは本日でございます。 In business, が/けれども is more neutral than のに, which can sound a bit complain-y.

Can I replace のに with けど/が? What changes?

のに: strong “despite” with an emotional tinge (frustration/surprise).
けど/が: neutral “but,” often softer. Example:
…届かないけど、締め切りは今日です。 = “…it hasn’t arrived, but the deadline is today.” (less emotive than のに)

Is のに only used to complain?

No. It signals a contrary-to-expectation contrast, which often carries emotion, but it can be neutral or even positive:

無料なのにおいしい。 = It’s tasty even though it’s free.

How do you read the kanji, and are there spelling variants for “deadline”?

締め切り（しめきり） is the common form.
Variants: 締切, 〆切 appear in signage/UI. Stick with 締め切り in general writing.

Do I need to mark the recipient with に (e.g., “to me”)?

Only if you need to specify it. The recipient can be omitted when it’s obvious.

Natural: メールがまだ届かない。
With recipient: 私にメールがまだ届かない。 / このアドレスにメールが届かない。 For parcels: 家に荷物が届かない。

Can I swap the clause order?

Yes. For emphasis on the contradiction, many would lead with the surprising part:

締め切りは今日なのに、参加登録のメールがまだ届かない。 Both orders are fine; the first clause gets more emphasis.

Does メール always mean email?

Usually it means email, but colloquially it can mean a message (SMS, app message). If you must be clear, say Eメール, 確認メール (confirmation email), or name the platform (e.g., LINEのメッセージ).

Why is there a comma before 締め切り?

It marks a natural pause between the のに clause and the main clause. It’s optional but standard in writing. The grammar doesn’t require it; it just aids readability.

What is the の in のに doing?

It’s the nominalizer の, turning the preceding clause into a noun phrase that に attaches to. So structurally it’s “in/ despite the fact that …,” similar to how けれども follows the plain form.