La mensajería llamó al servicio posventa.

Because al is the contraction of a + el.

The verb llamar in this meaning normally takes a before the person or thing being called:

• llamar a alguien
• llamar al servicio posventa

So here:

• a + el servicio posventa → al servicio posventa

This contraction is required in standard Spanish whenever a is followed by the masculine singular article el.

Llamó is the preterite form of llamar, third person singular:

• yo llamé
• tú llamaste
• él/ella/usted llamó

In this sentence, it means called in the sense of a completed past action:

• La mensajería llamó... = The courier company called...

It suggests a specific call that happened at a particular moment in the past.

Because mensajería is a feminine noun in Spanish, so it takes la.

In this sentence, mensajería most likely refers to a courier service / delivery company / messaging service, not the abstract idea of messaging in general.

So la mensajería can mean something like:

• the courier company
• the delivery service

It is feminine because the noun itself is feminine, not because of the gender of any person involved.

Here, mensajería probably means a courier or parcel delivery service/company.

Depending on context, mensajería can refer to:

• the business of delivering packages/messages
• a courier company
• a messaging service

In Spain, in a sentence like this, many learners will understand it as the courier company or the delivery service.

It does not usually mean text messaging here unless the context clearly points that way.

Servicio posventa means after-sales service or post-sale customer service.

It refers to the support a company gives after a product has been sold, for example:

• returns
• repairs
• warranty issues
• follow-up help

In English, you might sometimes naturally translate it as:

• after-sales service
• customer support
• post-sales service

The exact English wording depends on context.

Because posventa is functioning like a modifier after servicio.

So servicio posventa works a bit like:

• after-sales service

Here, posventa is not being used as a separate noun with its own article. It is describing the type of service.

This is common in Spanish with business or technical expressions, for example:

• servicio técnico
• atención telefónica
• centro médico

So servicio posventa is a fixed type of compound expression.

When llamar means to call someone/something, yes, it normally uses a:

• Llamé a mi madre.
• Llamaron al hotel.
• La mensajería llamó al servicio posventa.

But llamar can also mean to name or to be called, and then the structure changes:

• Se llama Marta. = Her name is Marta.
• Llamaron al perro Toby. = They named the dog Toby.

So in your sentence, llamó a is the telephone/contact meaning.

Usually, no. La mensajería most naturally refers to the courier service/company, not an individual delivery person.

If you wanted to say the delivery person / courier called, you would more likely say:

• El mensajero llamó al servicio posventa.
• La mensajera llamó al servicio posventa.

So the original sentence sounds more like a company or service acting as the subject.

Yes, Spanish could absolutely say just:

• Llamó al servicio posventa.

Spanish often omits the subject because the verb ending already shows the person and number.

But including La mensajería helps make clear who called. It is especially useful when:

• introducing new information
• avoiding ambiguity
• emphasizing the subject

So both are possible, but the full sentence is clearer.

In Spain, posventa is pronounced approximately:

• pos-BEN-ta

A few pronunciation notes:

• pos- has a clear s sound
• v and b are pronounced very similarly in Spanish
• the stress is on ven: posVENta

And the full phrase:

• servicio posventa

would be pronounced roughly:

• ser-BI-thyo pos-BEN-ta in much of Spain
• or ser-BI-syo pos-BEN-ta in areas without the th sound

In modern standard Spanish, posventa is widely accepted and common. You may also see postventa.

In many current style guides, forms with pos- are often preferred before consonants, so posventa is a very normal spelling.

For a learner, the important thing is:

• posventa = correct
• postventa = also seen

In Spain, posventa is very natural in business contexts.

Sometimes, but not always.

• servicio posventa = after-sales service, support after purchase
• atención al cliente = customer service more generally

So if the issue is specifically about something that happened after buying a product, servicio posventa is more precise.

If you mean general customer service, then atención al cliente may be better.

Example:

• La mensajería llamó al servicio posventa = the courier company called the after-sales department
• La mensajería llamó a atención al cliente = the courier company called customer service

Both can work, but they are not exactly identical.