I have been working on a program to validate email addresses (syntax only) to filter out obviously bad addresses from internal files/databases prior to loading into a listserv list. My questions are "what is a valid email address?", and "where do I go to find the authorative definition?" I have come across two mailbox address references in the Internet Standards: RFC821 (pp29-31), part of STD0010 SMTP Service Extensions. J. Klensin, N. Freed, M. Rose, E. Stefferud & D. Crocker. November 1995. (Format: TXT=23299 bytes) (Obsoletes RFC1651) (Also RFC821, RFC1869) and RFC822 (pp 10-11, 27, 44-47) in STD0011 Standard for the format of ARPA Internet text messages. D. Crocker. Aug-13-1982. (Format: TXT=109200 bytes) (Obsoletes RFC1653) (Also RFC0822) However, these definitions are different. Since RFC822 is extensively mentioned in the Listserv manuals, I originally wrote the program according to the RFC822 simple address type of "addr-spec", and further limiting the domain part to "domain-ref", excluding the more complex "domain-literal". This should allow the following special characters in the domain part: !#$%&'*+-/=?^_`{|}~ The test load via bulk op in the web interface happened to contain underscore in the domain part, which Listserv rejected as invalid. I tried a non-existent domain in WHOIS at http://www.networksolutions.com/cgi-bin/whois/whois/ which states that only alphanumeric and hyphen are valid. Where is this written? RFC821 states that sub-domains (between the dots) must begin with alpha, end with alphanumeric, and may contain alphanumeric and hyphen. So I tried that, and the program rejected addresses with sub-domains beginning with numeric ,which Listserv had previously accepted. I'm sorry about the length, but my two questions remain. Can anyone point me in the right direction?