This error can occur for the following reasons:
- The recipient address rejected your message: normally, it’s an error caused by an anti-spam filter. Your message has been detected and labeled as spam so you must ask the recipient to whitelist you. A full list of SMTP error codes can be found here.
- You have pasted in customer details (i.e. their name, email address, company) into the admin system and it contains hidden characters. You can check whether this is the case by manually entering the data into each field and then re-saving the customer record to see if the problem still occurs.
- Special characters such as tildas (~) & umlauts (ä) in the customer details (i.e. their name, email address, company) can cause this problem because some mail clients (server-tools and / or desktop tools) don’t support them and throw an ‘invalid email’ exception when you try to send an email to an address which contains these characters.