Пожалуйста, перешли письмо начальнице по электронной почте.

Перешли is the imperative for ты (informal singular: “you”). If you’re speaking politely or to more than one person, you’d normally use перешлите (imperative for вы).

• Informal: Пожалуйста, перешли…
• Formal / plural: Пожалуйста, перешлите…

It comes from the perfective verb переслать (“to forward / to send along”).

Imperative formation here:

• переслать → (perfective imperative, ты) перешли
• переслать → (perfective imperative, вы) перешлите

This is one of those cases where the stem changes (not a simple -й ending).

Perfective перешли focuses on completing the action once: “forward it (once), get it done.”

Imperfective пересылай (from пересылать) is more like:

• an ongoing process (“be forwarding…”),
• a repeated/habitual action (“forward (things) regularly”),
• or a less “completed” instruction.

In a typical one-time request like this, perfective перешли / перешлите is the natural choice.

письмо is in the accusative (direct object): “forward the letter/email.”

For neuter nouns like письмо, nominative and accusative look the same:

• Nominative: письмо
• Accusative: письмо

Because it expresses the recipient (“to whom?”). With verbs of sending/forwarding, the recipient is typically in the dative:

• переслать (что?) письмо (кому?) начальнице

So начальнице = “to (the) female boss.”

Male boss:

• Пожалуйста, перешли письмо начальнику… (dative of начальник)

If you want a more neutral/role-based option, you can often use a title or position name (which may still have grammatical gender), e.g. руководителю (“to the manager/leader”), директору (“to the director”).

по + dative is a very common way to express means/channel of communication.

• по (чему?) по́чте → dative singular of почта
  So по электронной почте = “by email / via email.”

Yes. Both can mean “by email,” but they feel a bit different:

• по электронной почте (common, “via email” as a channel)
• электронной почтой (instrumental: “by means of email”; also correct, sometimes a bit more “instrument-like”)

In everyday instructions, по электронной почте is extremely common.

When пожалуйста is used as a parenthetical politeness word (“please”) at the beginning, it’s often set off with a comma:

• Пожалуйста, перешли…

You may sometimes see it without a comma in informal writing, but the comma is standard and recommended.

It’s flexible:

• Пожалуйста, перешли письмо… (very common)
• Перешли, пожалуйста, письмо… (also very common in speech)
• Перешли письмо, пожалуйста, начальнице… (possible, slightly more marked)

All mean the same; placement mainly affects emphasis and rhythm.

It can mean either, depending on context. Since the sentence adds по электронной почте, here it strongly implies an email/message being forwarded.

(If you wanted to be extra explicit, you could say письмо (email) in context, but in Russian письмо is often enough.)

Common stresses:

• пожАлуйста
• переШли
• письмО
• начальнИце
• электрОнной
• пОчте

So: ПожАлуйста, переШли письмО начальнИце по электрОнной пОчте.