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
- Scientific notation
- 5.321 × 10^3
Roman numerals
Roman numerals as modelled here goes up to 3,999.
Binary
1 0 1 0 0 1 1 0 0 1 0 0 1
1 × 4,096 + 0 × 2,048 + 1 × 1,024 + 0 × 512 + 0 × 256 + 1 × 128 + 1 × 64 + 0 × 32 + 0 × 16 + 1 × 8 + 0 × 4 + 0 × 2 + 1 × 1 = 5,321
Octal
1 2 3 1 1
1 × 4,096 + 2 × 512 + 3 × 64 + 1 × 8 + 1 × 1 = 5,321
Hexadecimal
1 4 c 9
1 × 4,096 + 4 × 256 + c × 16 + 9 × 1 = 5,321
Read as powers of two: 4,096 + 1,024 + 128 + 64 + 8 + 1 = 5,321.
In historical numeral systems
Egyptian
5 lotuses, 3 coils of rope, 2 hobbles, 1 stroke
5 × 1,000 + 3 × 100 + 2 × 10 + 1 × 1 = 5,321. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
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: 15 July 2026