Save Emails: Limitations and requirements

Your labels or sub-labels must not have “/“, “+“, “\“, special characters, or emojies in its name. For example, if your label name is “Inbox/Clients” please rename that label to “Inbox – Clients”.
Your labels or sub-labels must not have ” ” at the end or beginning of its name. For example, if your label name is “Support “ please rename that label to “Support”.
The label must have Show in IMAP enabled
The number of emails in the label (excluding sub-labels) should be limited to less than 10K. If you have more than 10K emails in the label you can organize them into sub-labels. For example, if you label “Support” has 130K emails then you can create sublabel “Support/Archive” where you can put all emails older than a certain date. This limitation is not about total number emails in the label (including sub-labels) but about number of email directly under the label.
The reason for this suggestion is that sync of delete and move operations for a label which has a large number of email messages can be quite slow (delays up to 15 minutes). You can have more than 10K emails in the label but please understand delete and move operations might take some time to replicate.