Message field support

Compare the SDK's declared provider profiles for headers, attachments, metadata, scheduling and tags.

Use this table to shortlist providers, then validate the complete message against the configured Layer. It describes the SDK’s exported base capability profiles, not every feature advertised by the remote service. Additional provider rules can refuse an otherwise supported field.

Provider	Headers	Attachment MIME selection	Inline	Metadata	Scheduling	Maximum tags
Resend	Unique	Both	Yes	No	Yes	75
Postmark	Repeated	Explicit	Yes	Yes	No	1
SendGrid	Unique	Both	Yes	Yes	Yes	10
Mailgun	Repeated	Explicit	Yes	Yes	Yes	3
MailerSend	Repeated	ProviderSelected	Yes	No	Yes	5
MailPace	Unique	Explicit	Yes	No	No	75
Unosend	Unique	Explicit	No	No	Yes	75
Lettermint	Unique	Both	Yes	Yes	Yes	20
Plunk	Unique	Explicit	Yes	No	No	0
Helo	Unique	Both	Yes	Yes	No	5
JetEmail	Unique	ProviderSelected	No	No	Yes	0
Lettr	Unique	Explicit	No	Yes	No	1
SparkPost	Unique	Explicit	Yes	Yes	Yes	0
Primitive	Unique	Both	Yes	No	No	0
Mailtrap	Unique	Both	Yes	Yes	No	1
Mailchimp Transactional	Unique	Explicit	Yes	Yes	Yes	0
Scaleway	Unique	Explicit	No	No	No	0
ZeptoMail	Unique	Explicit	Yes	No	No	0
Cloudflare Email Sending	Unique	Explicit	Yes	No	No	0
Amazon SES	Unique	Explicit	Yes	No	No	50
Microsoft Graph	Unique	Explicit	Yes	No	No	0
SMTP	Repeated	Explicit	Yes	No	No	0
Brevo	Unique	ProviderSelected	No	No	Yes	0
Sequenzy	None	Both	No	No	No	0
SendHeron	Unique	Explicit	No	No	Yes	0

Read the table

Unique headers require distinct names ignoring case. Repeated preserves repeated names. A provider can additionally restrict which names are allowed.
Explicit attachment MIME selection requires a concrete content type. ProviderSelected delegates selection. Both supports either choice, never silent conversion between them.
A supported schedule still has provider-specific precision and time-window checks. Inline attachments can have additional CID or filename restrictions.
Tag counts do not describe allowed label syntax. Metadata support does not imply that nested application objects can be sent.

Account and mode differences

Lettr’s base profile is Immediate; Scheduled mode enables scheduling and removes key support. Mailchimp’s base profile disables tags; its explicit known-tag statistics configuration can enable a bounded subset. SendHeron’s Disabled attachment mode removes attachment support and changes replay guarantees.

Use the acquired email.capabilities and email.personalization for the configured service. The table does not cover Loops or Iterable native jobs, and field support alone never grants idempotent replay.