Seine E-Mail klingt formell.

Because E-Mail is a feminine noun in German: die E-Mail.

The possessive sein- means his, but it has to take an ending that matches the noun it describes.
For a feminine noun in the nominative singular, you use -e:

• sein Vater = his father
• seine Mutter = his mother
• seine E-Mail = his email

So seine E-Mail means his email.

E-Mail is usually feminine: die E-Mail.

That is why you get:

• die E-Mail
• eine E-Mail
• meine E-Mail
• seine E-Mail

You may occasionally see different usage in some regions, but for standard German, die E-Mail is the normal form learners should use.

Because all nouns are capitalized in German.

So in this sentence:

• Seine = capitalized because it is the first word of the sentence
• E-Mail = capitalized because it is a noun
• klingt = not capitalized
• formell = not capitalized, because it is an adjective here

Literally, klingen means to sound. But in German, just like in English, it is also used more broadly for how something comes across.

So Seine E-Mail klingt formell means something like:

• His email sounds formal
• His email comes across as formal

Even though an email is written, German still uses klingen here, just as English uses sounds in sentences like That sounds polite.

Because formell is not directly describing a noun here. It is used as a predicate adjective after the verb klingt.

In German:

• Attributive adjective before a noun: it takes an ending
  
  • eine formelle E-Mail = a formal email
• Predicate adjective after verbs like sein, wirken, bleiben, klingen: no ending
  
  • Die E-Mail ist formell
  • Die E-Mail klingt formell

So formell stays in its basic form.

Yes. That is grammatically correct.

There is a small difference in nuance:

• Seine E-Mail ist formell = his email is formal
• Seine E-Mail klingt formell = his email sounds formal

With klingt, the speaker is commenting on the impression the email gives. With ist, the statement sounds a bit more direct and definite.

This is a normal German main clause word order:

• Seine E-Mail = subject
• klingt = conjugated verb
• formell = complement

In a standard German main clause, the conjugated verb goes in the second position. Here the subject comes first, so the verb follows it:

• Seine E-Mail | klingt | formell

This is very similar to English word order in this sentence.

The infinitive is klingen = to sound.

klingt is the 3rd person singular form, used with er/sie/es and with singular nouns like die E-Mail.

Present tense:

• ich klinge
• du klingst
• er/sie/es klingt
• wir klingen
• ihr klingt
• sie/Sie klingen

Since E-Mail is singular, you use klingt.

Usually, yes, but there is one thing to watch out for.

sein / seine normally means:

• his
• sometimes its (for a neuter noun owner)

Examples:

• sein Hund = his dog
• seine Tasche = his bag
• Das Kind hat seine Jacke. = The child has its jacket.

In your sentence, Seine E-Mail most naturally means his email.

Yes, but the wording changes slightly.

Examples:

• Seine Mail klingt formell.
  Very common in everyday German, a bit less formal.
• Seine Nachricht klingt formell.
  Means his message sounds formal, which is broader than just email.
• Seine E-Mail klingt formell.
  Specific and very natural.

So E-Mail is a clear and standard choice here.

A rough guide for English speakers:

• Seine ≈ ZINE-uh
• E-Mail ≈ AY-mail
• klingt ≈ klingkt (with a short i)
• formell ≈ for-MELL

A few pronunciation notes:

• s at the start of Seine sounds like z
• ei sounds like English eye
• E in E-Mail is pronounced like ay
• formell has stress on the second syllable: for-MELL

Not necessarily. It simply means formal.

Depending on context, that can be:

• positive: polite, professional, respectful
• neutral: not casual
• slightly negative: stiff, distant, overly official

So Seine E-Mail klingt formell does not automatically mean criticism. It just describes the style.

Common opposites would be:

• Seine E-Mail klingt informell. = His email sounds informal.
• Seine E-Mail klingt locker. = His email sounds casual.
• Seine E-Mail klingt umgangssprachlich. = His email sounds colloquial.

The most direct opposite of formell is informell.