Obratite se zračnom prijevozniku e-mailom.

It is the imperative form of the verb obratiti se.

In this sentence, obratite se means something like contact / address / turn to. The ending -te shows that it is:

• 2nd person plural: talking to more than one person, or
• formal singular: talking politely to one person

So this is the kind of form you often see in instructions, notices, and customer-service language.

Se is a reflexive clitic, and it is part of the verb obratiti se.

That matters because:

• obratiti without se has a different meaning, such as turn something around or direct something
• obratiti se means turn to someone / address someone / contact someone

So in this sentence, se is not optional. It belongs to the verb.

Yes. Obratite se can be used for:

• more than one person
• one person formally/politely

The informal singular version would be:

• Obrati se = Contact... (said to one person informally)

So if a website, airline, office, or official notice is speaking to the public, obratite se is very natural.

Because se is a clitic, and Croatian clitics usually cannot stand at the beginning of a sentence.

So:

• Obratite se... = normal
• Se obratite... = not correct in normal standard word order

Croatian clitics usually go in the second position of the clause, so after the first stressed element you often get se.

Because obratiti se takes the dative case.

You ask:

• obratiti se komu? čemu? = turn/address oneself to whom? to what?

So the basic dictionary form is:

• zračni prijevoznik = air carrier / airline

But after obratiti se, it becomes dative singular:

• zračnom prijevozniku

Both words change because the adjective must agree with the noun:

• zračni → zračnom
• prijevoznik → prijevozniku

Literally, it means air carrier.

That is a formal or official way to say airline. In practical usage, it refers to the company that operates the flight.

Learners may also see other expressions, such as:

• aviokompanija = airline
• avio-prijevoznik = air carrier

But zračni prijevoznik sounds especially official or administrative.

Because it is in the instrumental case.

Here the instrumental expresses the means or method: by email.

So:

• e-mail = dictionary form
• e-mailom = by email

This is very common in Croatian:

• telefonom = by phone
• poštom = by mail
• autobusom = by bus

So e-mailom is the same pattern.

Because Croatian often uses the instrumental case without a preposition to express means or method.

English needs by:

• by email

Croatian can just use the instrumental:

• e-mailom

So the idea of by is built into the case ending here.

You can also say things like:

• putem e-maila = via email
• e-poštom = by email / by electronic mail

But e-mailom is short and natural.

All of these may be seen, but they differ a bit in style.

• e-mailom = very common
• emailom = also seen
• e-poštom = often felt to be more Croatian / more formal / more standard in some contexts

In everyday use, e-mailom is widely understood and very common. In very formal or language-conscious contexts, you may also encounter e-poštom.

Yes, that is also possible.

But the grammar changes:

• obratiti se komu → takes dative
• kontaktirati koga → takes a direct object in the accusative

So you get:

• Obratite se zračnom prijevozniku e-mailom.
• Kontaktirajte zračnog prijevoznika e-mailom.

The first sounds a bit more like turn to / get in touch with / address yourself to, which is common in instructions and official language. The second is a more direct contact.

Because obratiti se is the perfective verb, while obraćati se is its imperfective partner.

• obratiti se = make contact / turn to someone as a completed action
• obraćati se = be turning to / keep turning to / do it repeatedly or generally

In an instruction telling someone what step to take, the perfective imperative obratite se is very natural.

Compare:

• Obratite se službi za korisnike. = Contact customer service.
• Za sva pitanja obraćajte se službi za korisnike. = For all questions, address/contact customer service.
  This can sound more habitual or general.

So in your sentence, obratite se fits the one-step instruction very well.