Today one of my customers reported an error while sending emails from Outlook / Outlook Express, after investigating Exim logs I found this:
Dropped: too many syntax or protocol errors
SMTP connection from [18.104.22.168]:63138 (TCP/IP connection count = 3) SMTP call from (PEMVAR54) [22.214.171.124]:63138 dropped: too many syntax or protocol errors (last command was "RCPT TO: <'firstname.lastname@example.org'>")
This error can be also related with an error code: (0x800CCC0F): ‘The connection to the server was interrupted. If this problem continues, contact your server administrator or ISP. The server responded: 501 Too many syntax or protocol errors’
How can I fix it?
When you are sending your emails, ensure you are copying and pasting the email address ok, ensure it doesn’t have any special characters like quotations around the email address, as this is interpreted as a strange character and your Email server doesn’t know how to handle it.
In clear words, type the email again, ensure there are no weird symbols or characters and that’s all, it should be working.