sendmail, 4th Edition

BlankSub

Set unquoted space replacement character All versions

Some mailer programs have difficulty handling addresses that contain spaces. Such addresses are both illegal under RFC2821 and RFC2822 and subject to gross misinterpretation. For example, the address:

John Q [e-mail]        ← decidedly not kosher

is viewed by some MUA programs as being composed of three separate addresses: John, Q, and [e-mail]. To prevent this misinterpretation, such MUAs usually either quote the user portion or escape each space with a backslash:

"John Q Public"@wash.dc.gov        ← quoted
John\ Q\ [e-mail]        ← escaped

The BlankSub option is intended to handle an address that contains internal spaces, and is neither quoted nor escaped. For sendmail, a space is any character defined by the C-language library routine isspace(3).

Most sites use a . (dot or period) or an _ (underscore) character to replace unquoted space characters. That is, they declare the BlankSub option as one of the following:

O BlankSub=.
O BlankSub=_

Feeding the address:

John Q [e-mail]

through sendmail with the option BlankSub set to a dot yields:

[e-mail]

The forms of the BlankSub option are as follows:

O BlankSub=char                ← configuration file (V8.7 and later)
-OBlankSub=char                ← command line (V8.7 and later)
define(`confBLANK_SUB',char)   ← mc configuration (V8.7 and later)
OBchar                         ← configuration file (deprecated)
-oBchar                        ← command line (deprecated)

The argument char is of type character and is a single character. ...