Internet Message Access Protocol (IMAP) Capabilities Registry

Last Updated
2026-02-27
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Registries Included Below

IMAP Capabilities

Registration Procedure(s)
IETF Review
Reference
[RFC3501][RFC9051]
Note
Client implementations compliant with [RFC3501] SHOULD NOT require any 
capability name other than "IMAP4rev1". Client implementations 
compliant with [RFC9051] SHOULD NOT require any capability name other 
than "IMAP4rev2", and possibly "STARTTLS" and "LOGINDISABLED" (on a 
cleartext port). All client implementations MUST ignore any unknown 
capability names.

The prefix "QUOTA=RES-" is reserved per [RFC9208, Section 9.1].
See Section 9.2 of that document for values that follow this prefix.

All other capabilities starting with the "QUOTA=" prefix are reserved 
for future IETF Stream extensions to [RFC9208].
Available Formats

CSV

IMAP Quota Resource Types

Registration Procedure(s)
Specification Required
Expert(s)
Alexey Melnikov, Ken Murchison
Reference
[RFC9208]
Available Formats

CSV
Name of the Quota Resource Type Description Extra required IMAP commands/responses Extra optional IMAP commands/responses Author Change Controller Reference
STORAGE The physical space estimate, in units of 1024 octets, of the mailboxes governed by the quota root. DELETED-STORAGE STATUS request data item and response data item N/A [Alexey_Melnikov] [IESG] [RFC9208, Section 5.1]
MESSAGE The number of messages stored within the mailboxes governed by the quota root. DELETED STATUS request data item and response data item N/A [Alexey_Melnikov] [IESG] [RFC9208, Section 5.2]
MAILBOX The number of mailboxes governed by the quota root. N/A N/A [Alexey_Melnikov] [IESG] [RFC9208, Section 5.3]
ANNOTATION-STORAGE The maximum size of all annotations [RFC5257], in units of 1024 octets, associated with all messages in the mailboxes governed by the quota root. N/A N/A [Alexey_Melnikov] [IESG] [RFC9208, Section 5.4]

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