Reasons Gmail May Send Messages to Spam Folder¶
Gmail is notoriously strict when it comes to spam prevention for their email accounts. As such, it is not uncommon for legitimate messages sent to their servers to be blocked, entirely, or end up marked as spam and sent to the recipient’s spam folders. The following are some steps that may be taken to prevent this from occurring.
More detailed information from Google can be found here: https://support.google.com/mail/answer/81126?hl=en.
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Other Considerations¶
Even if a domain or server has all of the email-related DNS records that Gmail looks for, there are several other reasons that legitimate messages may be sent to the recipient’s spam folder. Unfortunately, many of these causes are not easily controllable and it may just take time for them to resolve themselves.
The more messages marked by the recipients as “not spam”, the more likely it is for Gmail to eventually allow messages from the domain through to the inbox.
Other potential causes:
The domain, hostname, or IP has been listed on an RBL
Even if it is no longer listed, Gmail may still have the past listing information cached
The domain has recently been added to the server, or has recently changed IP address, and has not yet built up positive reputation
Messages from the domain have been marked as spam by recipients in the past
The message looks like other messages that were marked as spam in the past
If the message has a subject, wording, or certain phrases that were included in other messages that were marked as spam, this can trigger Gmail’s spam filters. This includes all messages that were ever sent to Gmail’s servers, from any sending domain or IP address, not just messages sent to the recipient