SMS and MMS message limits, sender ID rules, what you cannot send, and the errors you will meet, each with its fix.

Limits

Limit	Value
Text	At most 10 segments (1,530 GSM-7 or 670 unicode characters)
Segment size	GSM-7: 160 characters, or 153 per part of a multi-part message. UCS-2 (emoji, non-Latin scripts): 70, or 67 per part
MMS media	Up to 10 public `mediaUrls` per message; `text` is optional
Broadcast attachments	Up to 10 public http(s) URLs with no redirects; JPEG, PNG, GIF, WEBP, MP4 or 3GPP under 1 MB
Scheduling	`sendAt` as ISO 8601 with offset, in the future
Opt-out list	`limit` defaults to 500, at most 5,000, most recent first, no cursor
Destinations	Only the destinations in the SMS price list; others are blocked at send time
Sender ID name	3 to 11 characters: letters, digits, spaces, at least one letter
Sender IDs per team	1,000 active
Sender ID daily budget	500 messages a day by default (level 1), then 2,000, 10,000 or 25,000 on request; shared across names, reset at midnight UTC
Sole-proprietor OTP	A new code at most once a minute
Opt-in proof upload	PNG, JPG, WebP, GIF or PDF, up to 4 MB
Hosted registration link	Single use, valid 7 days

What you cannot do

SMS on Zernio does not allow:

Delivering from a US number before its carrier registration is approved
Texting from a number that is not SMS-capable (enabled: false with smsCapable: false)
Enabling SMS on WhatsApp numbers you brought through embedded signup
Messaging a recipient who replied STOP (only they can re-subscribe, by replying START)
Receiving replies on a sender ID
Sending MMS from a sender ID
Sending from a sender ID to the US, Canada or Puerto Rico
Claiming a sender ID name for your team alone (names are not exclusive)
Appealing a brand-level rejection (correct the brand details and re-verify instead)
Appealing a toll-free verification (/appeal covers 10DLC campaigns only)
Quoting an SMS price before sending (messages are carrier-rated after they are sent)

Status	Code	Cause	Fix
`400`	`missing_required_field`	A registration is missing a required field, named in `param`	Add the field and resubmit; run `POST /v1/sms/registrations/preflight` to catch every gap
`422`		The carrier registry rejected a registration field; `param` names it when known	Correct that field and resubmit
`404`		No active SMS-enabled number matches `from`: you do not own it, or its US registration is not approved	Send from a number you own; wait for `approved`
`409`		The recipient replied STOP	Drop them from your list; they come back only by replying START
`409`		The same `Idempotency-Key` is still in flight	Wait for the first request to finish
`422`		The `Idempotency-Key` was reused with a different body	Use a new key for a new message
`400`		A broadcast attachment is on a private host, redirects, is over 1 MB, or there are more than 10	Fix the attachment list
`403`	`alpha_destination_not_supported`	A sender ID send to the US, Canada or Puerto Rico	Send from a phone number
`403`	`mms_not_supported`	`mediaUrls` on a sender ID send	Remove the media, or send from a number
`429`	`alpha_daily_limit_reached`	Today's sender ID budget is spent	Wait for the midnight UTC reset, or request a higher level
`403`	`sender_id_limit_reached`	The team holds 1,000 active sender IDs	Contact support to raise it
`422`		The sender ID name impersonates a well-known brand or institution	Use your own brand name
`402`		No payment method is on file	Add a payment method

A message that fails after it was accepted fires message.failed with the carrier's error code.

Every error uses the envelope in error handling.

Limit	Value
Text	At most 10 segments (1,530 GSM-7 or 670 unicode characters)
Segment size	GSM-7: 160 characters, or 153 per part of a multi-part message. UCS-2 (emoji, non-Latin scripts): 70, or 67 per part
MMS media	Up to 10 public `mediaUrls` per message; `text` is optional
Broadcast attachments	Up to 10 public http(s) URLs with no redirects; JPEG, PNG, GIF, WEBP, MP4 or 3GPP under 1 MB
Scheduling	`sendAt` as ISO 8601 with offset, in the future
Opt-out list	`limit` defaults to 500, at most 5,000, most recent first, no cursor
Destinations	Only the destinations in the SMS price list; others are blocked at send time
Sender ID name	3 to 11 characters: letters, digits, spaces, at least one letter
Sender IDs per team	1,000 active
Sender ID daily budget	500 messages a day by default (level 1), then 2,000, 10,000 or 25,000 on request; shared across names, reset at midnight UTC
Sole-proprietor OTP	A new code at most once a minute
Opt-in proof upload	PNG, JPG, WebP, GIF or PDF, up to 4 MB
Hosted registration link	Single use, valid 7 days

SMS on Zernio does not allow:

Delivering from a US number before its carrier registration is approved
Texting from a number that is not SMS-capable (enabled: false with smsCapable: false)
Enabling SMS on WhatsApp numbers you brought through embedded signup
Messaging a recipient who replied STOP (only they can re-subscribe, by replying START)
Receiving replies on a sender ID
Sending MMS from a sender ID
Sending from a sender ID to the US, Canada or Puerto Rico
Claiming a sender ID name for your team alone (names are not exclusive)
Appealing a brand-level rejection (correct the brand details and re-verify instead)
Appealing a toll-free verification (/appeal covers 10DLC campaigns only)
Quoting an SMS price before sending (messages are carrier-rated after they are sent)

Status	Code	Cause	Fix
`400`	`missing_required_field`	A registration is missing a required field, named in `param`	Add the field and resubmit; run `POST /v1/sms/registrations/preflight` to catch every gap
`422`		The carrier registry rejected a registration field; `param` names it when known	Correct that field and resubmit
`404`		No active SMS-enabled number matches `from`: you do not own it, or its US registration is not approved	Send from a number you own; wait for `approved`
`409`		The recipient replied STOP	Drop them from your list; they come back only by replying START
`409`		The same `Idempotency-Key` is still in flight	Wait for the first request to finish
`422`		The `Idempotency-Key` was reused with a different body	Use a new key for a new message
`400`		A broadcast attachment is on a private host, redirects, is over 1 MB, or there are more than 10	Fix the attachment list
`403`	`alpha_destination_not_supported`	A sender ID send to the US, Canada or Puerto Rico	Send from a phone number
`403`	`mms_not_supported`	`mediaUrls` on a sender ID send	Remove the media, or send from a number
`429`	`alpha_daily_limit_reached`	Today's sender ID budget is spent	Wait for the midnight UTC reset, or request a higher level
`403`	`sender_id_limit_reached`	The team holds 1,000 active sender IDs	Contact support to raise it
`422`		The sender ID name impersonates a well-known brand or institution	Use your own brand name
`402`		No payment method is on file	Add a payment method

A message that fails after it was accepted fires message.failed with the carrier's error code.

Every error uses the envelope in error handling.