SMTP Error 501 : The command was correct and
recognised, but the parameters (the arguments, e.g. email
address) were not valid.
For example, the following email address will definitely give
an SMTP Error 501 with most mail servers,
happy\firstname.lastname@example.org, as “\” is not allowed in email
addresses, which makes this email address invalid.
In the vast majority of cases SMTP Error 501 is caused
by invalid email addresses. For example, a typical return
error message might be : “<remote-server-ip-address>
does not like recipient. Remote host said: 501 Invalid
In cases where the error is not caused by an invalid email
address, an SMTP Error 501, particularly if repeated, can
be indicative of communications problems, such as a noisy
line, intermittent drops in network connections, etc…
Information above taken from here.