El buzón de voz es útil.

Because buzón is a masculine noun in Spanish, so it takes the masculine singular article el.

• el buzón = the mailbox / voicemail box
• la is only used with feminine singular nouns

You usually have to learn the gender of each noun as part of the word.

Literally, buzón de voz means voice mailbox.

It is made up of:

• buzón = mailbox / box
• de = of
• voz = voice

So Spanish uses of where English often uses a noun + noun structure:

• buzón de voz = voicemail
• literally: mailbox of voice

This noun + de + noun pattern is very common in Spanish.

Spanish often links nouns with de where English simply puts one noun before another.

Compare:

• voice mail / voicemail in English
• buzón de voz in Spanish

Other similar examples:

• tarjeta de crédito = credit card
• jugo de naranja = orange juice
• sala de estar = living room

So de is the normal way to connect these ideas in Spanish.

Because ser is normally used for general characteristics or essential qualities, and estar is more often used for states, conditions, or locations.

Here, útil describes a general quality of voicemail:

• El buzón de voz es útil. = Voicemail is useful.

That is why es is the natural choice.

Using estar útil would sound wrong in this sentence.

In Spanish, adjectives often come after the noun, especially when they describe a basic quality.

So:

• el buzón de voz útil = the useful voicemail box
• El buzón de voz es útil. = The voicemail is useful.

In this sentence, útil comes after es because it is part of the predicate, not directly attached before the noun.

English often puts adjectives before nouns, but Spanish usually does not.

Útil stays the same in the masculine singular and feminine singular.

Examples:

• El buzón es útil.
• La herramienta es útil.

But in the plural, it changes to:

• útiles

Examples:

• Los buzones son útiles.
• Las herramientas son útiles.

So útil does agree in number, but not usually in masculine/feminine form.

The written accent in útil shows where the stress goes:

• Ú-til

Without the accent, Spanish pronunciation rules would suggest a different stress pattern. The accent mark tells you that the stress falls on the first syllable.

So:

• útil = stressed on ú

Accent marks in Spanish are very important because they often show correct pronunciation.

Buzón has an accent because the stress falls on the last syllable:

• bu-ZÓN

Words ending in n, s, or a vowel are normally stressed on the second-to-last syllable unless there is a written accent. Since buzón is stressed on the last syllable, it needs the accent mark.

A simple pronunciation guide is:

el boo-THON de BOTH es OO-til
in most of Spain

A few notes for Spain Spanish:

• z in buzón sounds like the th in think
• c before e/i would also have that sound in most of Spain, but there is no c here
• v and b are pronounced very similarly in Spanish
• voz ends with the same th sound in most of Spain

A more detailed breakdown:

• El = ehl
• buzón = boo-THON
• de = deh
• voz = both
• es = ess
• útil = OO-teel

In much of Latin America, the z would sound like s instead.

Because el belongs to buzón, not to voz.

The noun phrase is:

• el buzón = the mailbox
• de voz = of voice

So the main noun is buzón, which is masculine.
Voz is feminine as a word, but it does not control the article here.

You can see this more clearly with another article:

• la voz = the voice
• but el buzón de voz = the voicemail box

Yes, in many contexts correo de voz is also used for voicemail.

However, buzón de voz is a very common expression, especially when referring to the voicemail box or voicemail service itself.

The exact preferred term can vary by region, company, or context, but El buzón de voz es útil is perfectly natural.

Because in expressions like buzón de voz, Spanish often leaves out the article after de when the second noun is being used in a general or classifying way.

So:

• buzón de voz = voicemail box
• not usually buzón de la voz

Adding la would change the meaning and make it sound like a specific voice, which is not the idea here.

This is common in Spanish:

• copa de vino = glass of wine
• zapatos de cuero = leather shoes
• caja de madera = wooden box

The plural would be:

Los buzones de voz son útiles.

Changes:

• el → los
• buzón → buzones
• es → son
• útil → útiles

Notice that voz stays singular here because the type of thing is still voice mailboxes, not necessarily boxes of voices.

Normally, no. In Spanish, singular countable nouns usually need an article or another determiner.

So the natural sentence is:

• El buzón de voz es útil.

Without el, it sounds incomplete in standard Spanish.

No. Buzón is a general word meaning mailbox, postbox, or sometimes any kind of message box.

For example:

• buzón de correo = mailbox / email inbox, depending on context
• buzón de sugerencias = suggestion box
• buzón de voz = voicemail box

So the meaning depends on the words that follow it.