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from is what the recipient sees as the sender.

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The default is the trap

Omitting from uses your login name. That is convenient in testing and almost never right in production, because your login name is usually not an approved sender ID — so the same code that worked in staging returns 403 the moment approval is enforced.
Set from explicitly from the very first message, even while approval is off. It costs nothing and removes an entire class of go-live surprise.

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Two kinds of sender ID

Alphanumeric is typically limited to 11 characters, and support varies by country — some destinations refuse it entirely, some overwrite it, and some require pre-registration. Your provider knows which applies to your traffic.

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When approval is enforced

On an account with sender-ID whitelisting on, an unapproved from is refused:
403 over HTTP, 0x415 over SMPP. Nothing was sent and nothing was charged. This is not fixable in code. It needs an approval from whoever runs your gateway, usually requested through the customer portal.

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Requesting one

1

Ask for the exact string

SUNOTP and SunOTP are different sender IDs. Ask for the casing you will actually send.
2

Say which destinations it is for

Regulated destinations — India most visibly — need registration with the carrier as well as with your gateway, and that takes longer.
3

Wait for APPROVED

A sender ID that is pending is not usable. It behaves exactly as if it did not exist.
An approved sender ID may also carry TLVs, which are stamped onto your message when it matches. On a DLT lane that stamp is the registered entity id — so an unapproved sender is not only refused for being unapproved, it would also have arrived at the carrier without the parameters it needed.

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Sender ID and content approval are separate

Two different checks, two different refusals: An account may enforce either, both, or neither. Passing one says nothing about the other — see Refusals for reading the second.

Sender IDs and templates

The operator’s side: approving, rejecting and what a stamp does.

Errors

What to do with a 403 that will not go away.