5321
RFC 5321 is the current Simple Mail Transfer Protocol specification.
- Odd
- Composite
Where 5321 fits
Why 5321 is special
It replaced RFC 821 after twenty-six years and still keeps the original's permissive envelope, because changing it would break every mail server at once.
5321 is part of
Factors & digit properties
- Divisors (4)
- 1, 17, 313, 5321
- Prime factorisation
- 17 × 313
- Digit sum
- 11
- Digital root
- 2
Representations
- Decimal
- 5321
- Binary
- 1010011001001
- Octal
- 12311
- Hexadecimal
- 14c9
- Scientific notation
- 5.321 × 10^3
Other mathematical patterns
- Deficient number
- Semiprime
Related numbers & connections
Mail transfer keeps the permissive envelope of its 1982 predecessor, because changing it would break every server at once.
RFC 5321 replaced RFC 821 after twenty-six years while keeping the permissive envelope, because changing it would break every mail server simultaneously.
View 821 →Numbers Around Us
- appears inNumbers that make the internet work
why?
Strength 0.50, confidence "high".
Other connections
- developed from821
why?
Strength 0.50, confidence "high".
Technology & computing
RFC 5321 obsoletes RFC 2821 and RFC 821, consolidating extensions such as size declaration and pipelining while preserving backwards compatibility with the original envelope model.
DocumentedTechnology & computingGeography: International — IETFPeriod: 2008
Common questions
Is 5321 a prime number?
No, 5321 is not prime.
What are the factors of 5321?
The factors of 5321 are 1, 17, 313, 5321.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 3 August 2026