Quando eu souber o horário do voo, vou lhe enviar uma mensagem.

Because Portuguese uses the future subjunctive after quando when you are talking about a future situation.

So:

• quando eu souber = when I find out / when I know
• quando eu sei would not work here

In English, we usually use the present after when: When I know the flight time...
In Portuguese, that idea normally becomes quando eu souber...

Because after quando in this kind of future sentence, Portuguese needs a conjugated verb, not the infinitive.

• saber = infinitive, to know
• souber = future subjunctive, I know / find out in a future-dependent clause

So:

• correct: Quando eu souber...
• incorrect: Quando eu saber...

A rough pattern is:

• quando eu souber
• quando você souber
• quando ele/ela souber
• quando nós soubermos
• quando vocês souberem
• quando eles souberem

It comes from the verb saber in the future subjunctive.

A useful shortcut is that the future subjunctive is often based on the third-person plural preterite form:

• preterite: eles souberam
• remove -am → souber-
• add the endings of the future subjunctive:
  • eu souber
  • você/ele/ela souber
  • nós soubermos
  • vocês/eles/elas souberem

So souber is not random; it follows a pattern.

Both are correct, but vou enviar is much more common in everyday Brazilian Portuguese.

• vou enviar = going to send
• enviarei = I will send

In Brazil, the ir + infinitive future is very natural in speech:

• vou enviar
• vou ligar
• vou falar

So this sentence sounds very normal and conversational.

Lhe means to you here.

In vou lhe enviar uma mensagem:

• uma mensagem is the thing being sent
• lhe is the person receiving it

So the structure is basically:

• enviar algo a alguém
• to send something to someone

That is why lhe is used: it works as an indirect object pronoun.

Yes. In Brazilian Portuguese, both are very common.

Possible versions:

• Vou lhe enviar uma mensagem.
• Vou te enviar uma mensagem.
• Vou enviar uma mensagem para você.

General feel:

• lhe = a bit more formal, written, or careful
• te = very common in informal speech
• para você = clear, natural, and very common

So the original sentence is correct, but it is not the only natural way to say it.

In Brazilian Portuguese, object pronouns often go before the infinitive in this kind of structure:

• vou lhe enviar
• quero te ajudar
• posso lhe mostrar

You may also see enviar-lhe, especially in more formal writing or in European Portuguese, but in Brazil vou lhe enviar sounds more natural in everyday use.

It means the time/schedule of the flight.

Depending on context, it could mean:

• the departure time
• the arrival time
• the scheduled time generally

Very often, horário do voo is understood as the flight time/schedule information. If someone wanted to be more specific, they could say:

• o horário de partida do voo = the departure time of the flight
• o horário de chegada do voo = the arrival time of the flight

Because do is a contraction of de + o.

So:

• de o voo → do voo

This is extremely common in Portuguese:

• de + o = do
• de + a = da
• em + o = no
• a + o = ao

So o horário do voo literally has the structure the time of the flight.

Because modern Portuguese spelling writes it as voo.

Older spelling used vôo, but that accent was removed in the spelling reform. So today the correct spelling is:

• voo

The same happened with some related forms, such as:

• enjoo
• perdoo

So if you see vôo in older texts, that is outdated spelling.

Because the sentence starts with a subordinate clause:

• Quando eu souber o horário do voo, = dependent clause
• vou lhe enviar uma mensagem. = main clause

In Portuguese, when this kind of clause comes first, a comma is normally used.

If you reverse the order, the comma usually disappears:

• Vou lhe enviar uma mensagem quando eu souber o horário do voo.

Yes. That is very natural.

Portuguese often drops subject pronouns when the verb already makes the subject clear. So both are possible:

• Quando eu souber o horário do voo...
• Quando souber o horário do voo...

Including eu can add a little emphasis or clarity, but it is not required here.

Not necessarily. Uma mensagem can be any kind of message, depending on context:

• a text message
• a WhatsApp message
• an email message
• a message passed along to someone

In everyday situations, many people will naturally understand it as a phone message or text if that fits the context. If you want to be more specific, you could say:

• uma mensagem de texto = a text message
• uma mensagem no WhatsApp = a WhatsApp message