There are several messages and errors that SendGrid provides to help you identify if your email is reaching your guests.
Each SMTP (Simple Mail Transfer Protocol) made returns a response.
200 responses are usually success responses, and
400 responses are usually deferrals. SendGrid continues to retry re-sending
400 messages for up to 72 hours.
500 responses are hard failures that are not retried by SendGrid servers.
This table has possible response codes with example errors and a general explanation of that sort of response. This is not an all-inclusive list, but has they are the most common:
|250||`Queued mail for delivery`||Your mail has been successfully queued! This response indicates that the recipient server has accepted the message.|
|421||`Message from (X.X.X.X) temporarily deferred`||Messages are temporarily deferred because of recipient server policy - often it’s because of too many messages or connections in too short of a time frame. We continue to retry deferred messages for up to 72 hours. Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried.|
|450||`too frequent connects from 184.108.40.206, please try again later.`||The message failed because the recipients mailbox was unavailable, perhaps because it was locked or was not routable at the time. We continue to retry messages for up to 72 hours. Consider temporarily sending less messages to a domain that is returning this code because this could further delay your messages currently being tried.|
|451||`Temporary local problem - please try later`||The message simply failed, usually due to a far-end server error. We continue to retry messages for up to 72 hours.|
|452||`Too many recipients received this hour (throttled)`||The message has been deferred due to insufficient system storage. We continue to retry messages for up to 72 hours.|
|550||`Requested action not taken: mailbox unavailable`||The user’s mailbox was unavailable. Usually because it could not be found, or because of incoming policy reasons. Remove these address from your list - it is likely a fake, or it was mis-typed.|
|551||`User does not exist.`||The intended mailbox does not exist on this recipient server. Remove these addresses from your list.|
|552||`This message is larger than the current system limit or the recipient’s mailbox is full. Create a shorter message body or remove attachments and try sending it again.`||The recipients mailbox has exceeded its storage limits. We don’t resend messages with this error code because this is usually a sign this is an abandoned email.|
|553||`Invalid/inactive user.`||The message was refused because the mailbox name is either malformed or does not exist. Remove these addresses from your list.|
|554||`ERROR: Mail refused`||This is a default response that can be caused by a lot of issues. There is often a human readable portion of this error that gives more detailed information, but if not, remove these addresses from your list.|
|Other||`Delayed Bounce - Unable to Parse Server Reason`||This is what SendGrid displays when the recipients server returns a blank reason code.|