The MAIL FROM: command of the SMTP transaction is used to convey the address of the envelope sender. RFC821 requires that the envelope sender address always be fully qualified. That is, it must always have a user part, an @ character, and a domain part, in that order.

The normal behavior of sendmail is to reject the envelope sender if it is not fully qualified. For example:

MAIL FROM: <you>
553 5.5.4 <you>... Domain name required

This rejection is done for network connections only. When reading the envelope sender via the standard input under the -bs command-line switch (-bs), a missing @domain part is OK:

% /usr/sbin/sendmail -bs
220 yourhost.domain ESMTP Sendmail 8.12.7/8.12.7; Fri, 13 Dec 2002 14:13:09 -0700
HELO yourhost
250 yourhost.domain Hello your@yourhost.domain, pleased to meet you
MAIL FROM: <bob>
250 2.1.0 <bob>... Sender ok

If machines at your site routinely send unqualified envelope sender addresses (addresses without the @domain part), you will find that mail is being rejected.

Your first attempt at a solution should be to fix the broken software that is sending unqualified addresses. If that fails, or if you lack the permission or authority, you can use this accept_unqualified_senders feature to force sendmail to accept unqualified envelope sender addresses:

FEATURE(`accept_unqualified_senders')

Another way to handle this problem is with the (V8.10 and above) DaemonPortOptions option's Modifier key value (See this section). If that value includes a u character, unqualified envelope sender addresses are accepted even if this feature is omitted. Even if this feature is included, the presence of an f in the DaemonPortOptions option's Modifier key value causes the normal behavior of enforcing fully qualified addresses.