What is the email subject length limit

Question

How many characters are allowed to be in the subject line of Internet email  I had a scan of The RFC for email but could not see specifically how long it was allowed to be  I have a colleague that wants to programmatically validate for it   If there is no formal limit  what is a good length in practice to suggest

User · Answer

What s important is which mechanism you are using the send the email  Most modern libraries  i e  System Net Mail   will hide the folding from you  You just put a very long email subject line in without  CR LF HTAB   If you start trying to do your own folding all bets are off  It will start reporting errors  So if you are having this issue just filter out the CR LF HTAB and let the library do the work for you  You can usually also set the encoding text type as a separate field  No need for iso encoding in the subject line

User · Answer

I don t believe that there is a formal limit here  and I m pretty sure there isn t any hard limit specified in the RFC either  as you found   I think that some pretty common limitations for subject lines in general  not just e-mail  are    80 Characters 128 Characters 256 Characters   Obviously  you want to come up with something that is reasonable  If you re writing an e-mail client  you may want to go with something like 256 characters  and obviously test thoroughly against big commercial servers out there to make sure they serve your mail correctly   Hope this helps

User · Answer

after some test  If you send an email to an outlook client  and the subject is  77 chars  and it needs to use    ISO  inside the subject  in my case because of accents  then OutLook will  cut  the subject in the middle of it and mesh it all that comes after  including body text  attaches  etc    all a mesh   I have several examples like this one   Subject    ISO-8859-1 Q Actas de la obra N BA 20100154  Expediente N BA 20100182   NUEVA RED FERROVIARIA    TRAMO 20BEASAIN 20OESTE Pedido 20PC10 00123-125   20BEASAIN       To   As you see  in the subject line it cutted on char 78 with a     followed by 2 or 3 line feeds  then continued with the rest of the subject baddly   It was reported to me from several customers who all where using OutLook  other email clients deal with those subjects ok   If you have no ISO on it  it doesn t hurt  but if you add it to your subject to be nice to RFC  then you get this surprise from OutLook  Bit if you don t add the ISOs  then iPhone email will not understand it and attach files with names using such characters will not work on iPhones

User · Answer

See RFC 2822  section 2 1 1 to start      There are two limits that this   standard places on the number of   characters in a line  Each line of   characters MUST be no more than 998   characters  and SHOULD be no more than   78 characters  excluding the CRLF    As the RFC states later  you can work around this limit  not that you should  by folding the subject over multiple lines      Each header field is logically a   single line of characters comprising   the field name  the colon  and the   field body   For convenience however    and to deal with the 998 78 character   limitations per line  the field body   portion of a header field can be split   into a multiple line representation    this is called  folding    The general   rule is that wherever this standard   allows for folding white space  not   simply WSP characters   a CRLF may be   inserted before any WSP   For    example  the header field          Subject  This is a test       can be represented as          Subject  This         is a test    The recommendation for no more than 78 characters in the subject header sounds reasonable   No one wants to scroll to see the entire subject line  and something important might get cut off on the right

User · Answer

RFC2322 states that the subject header  has no length restriction   but to produce long headers but you need to split it across multiple lines  a process called  folding    subject is defined as  unstructured  in RFC 5322  here s some quotes         indicate stuff i omitted   3 6 5  Informational Fields   The informational fields are all optional   The  Subject   and    Comments   fields are unstructured fields as defined in section   2 2 1         2 2 1  Unstructured Header Field Bodies   Some field bodies in this specification are defined simply as    unstructured   which is specified in section 3 2 5 as any printable   US-ASCII characters plus white space characters  with no further   restrictions   These are referred to as unstructured field bodies    Semantically  unstructured field bodies are simply to be treated as a   single line of characters with no further processing  except for    folding  and  unfolding  as described in section 2 2 3    2 2 3         An unfolded header field has no length restriction and   therefore may be indeterminately long

[email] What is the email subject length limit?

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