To expand on that. FileMaker's native Send Mail only supports sending email with a single attachment as you have found out. Other options are to send a separate email for each attachment or to zip the files to produce a single file before attaching.
And the MBS plug in recommended by Christian is just one of several email plug ins available that offer support for multiple attachments.
To be precise ..My error message reads.. after using functions
Set Variable [$Messages; MBS("CURL.GetDebugAsText"; $curl) ]
# check result
Set Variable [$Output; MBS("CURL.GetResultAsText"; $curl) ]
250 2.1.0 Sender OK
550 5.7.64 TenantAttribution; Relay Access Denied
RCPT failed: 550
221 2.0.0 Service closing transmission channel
Closing connection 0
...when I send it to someone in outlook ..script works ...