Instantly is one of the most popular tools for running cold email campaigns, and you can connect your Maildoso mailboxes to it in just a few steps. This guide walks you through the setup and helps you fix the errors people run into most often.
Before you start
Make sure your mailboxes are showing as Active in Maildoso. Newly created mailboxes can take a little time to finish setting up, and they can only be exported once they are active.
Have your Instantly login details handy — you'll connect using your Instantly login email and password, not an API key.
If your Instantly account has two-factor authentication (2FA) turned on, temporarily disable it before connecting, as it can block the connection.
Step 1: Connect your Instantly account
In Maildoso, click your profile picture in the top-right corner and open Settings.
Go to Sequencers, choose Instantly, and click Configure.
Enter your Instantly login email and password, then click Configure to save the connection.
Step 2: Add your mailboxes to Instantly
Open the Email Accounts section.
Click Export Accounts, then Add all Google Accounts to Sequencer.
Choose Instantly and confirm with Add to Sequencer.
Your mailboxes will begin connecting. This usually completes within 24 hours, after which they'll appear in your Instantly workspace ready to send.
Alternatively, you can log in to your Instantly AI account, navigate to Email Accounts, and click on Add New. Choose the IMAP and SMTP Import option or select the Bulk/CSV Import feature. Upload the CSV file exported from Maildoso to complete the import process.
"Request failed with status code 422"
This is the most common error people see when importing Maildoso SMTP mailboxes into Instantly. In most cases it does not mean the mailbox is broken or that it wasn't created.
What it usually is. The 422 comes from an IMAP validation check that failed at the moment of import. The mailbox itself is normally created correctly on our side, and an automated process on our end corrects the underlying IMAP issue shortly afterwards. It is also completely normal for only some rows in your file to fail while the rest import fine — that does not mean anything is wrong with those particular mailboxes or domains.
It can also mean you're over your plan's capacity. If you're importing more domains or mailboxes than your current plan actually covers (for example your plan includes 1 domain slot but you're trying to import mailboxes across 3 domains), the accounts beyond your covered amount will fail to import with a 422 as well. Check Settings → Billing (or your plan summary) against what you're trying to export — if you're over capacity, either reduce what you're exporting to match your plan or add the extra domains/mailboxes to your plan first.
What to do
Open your Email Accounts page and check the addresses that failed. If they show as Active, the mailboxes are fine — the error refers to the import attempt, not to the mailbox itself.
Wait a little while, then export a fresh CSV (Email Accounts → Export all to CSV) and import that new file into Instantly. Exporting again matters — an older file can still carry the details that failed the first time.
What won't help
Adding the failed mailboxes one by one in Instantly. If the IMAP check is failing, manual entry returns an authentication or IMAP connection error too.
Changing the SSL settings on IMAP or SMTP. That is not what causes a 422.
Re-uploading the same unchanged file straight away — give it a little time and export a fresh CSV first.
When to contact us. Get in touch if the same addresses still fail after you've waited and retried with a freshly exported CSV, or if those mailboxes are not showing as Active. Also note that an expired subscription, or having more mailboxes or domains than your plan covers, takes the affected accounts out of the active state — they can't be imported until your plan covers them again.
"Request failed with status code 409"
A 409 means Instantly sees a conflict — usually because the mailbox already exists in your Instantly workspace. It's not a sign that anything is broken on the Maildoso side.
Check your Instantly workspace for the mailbox(es) that failed. If they're already there, no further action is needed.
If they're not there, wait a little while (up to around 30 minutes), then try the import again.
Still failing after that? Contacting Instantly support directly can help, since this error originates on their side.
Mailbox connects fine elsewhere, but fails Instantly's SMTP test
Instantly runs its own SMTP verification check (sometimes surfaced via a "Gmass" test) when you add or import a mailbox. If a mailbox fails that specific check but otherwise connects and sends fine in another mail client (for example BlueMail or your regular email app), the mailbox itself is working — this is Instantly's verification step being stricter or timing out, not a sign your SMTP credentials are wrong. Try re-adding the mailbox after a short wait, and if it keeps failing only Instantly's test while working everywhere else, contact us with the mailbox address so we can check the SMTP delivery logs on our side, and consider also flagging it to Instantly support since the check runs on their end.
Instantly's "Slow Ramp-Up" toggle isn't the same as Maildoso's warmup limits
Instantly has its own Slow Ramp-Up setting (inside a campaign's sending options) that can cap a mailbox to a low number of sends per day — for example around 10/day — separately from anything on the Maildoso side. If your volume looks capped lower than expected even though your Maildoso warmup/sending limits are set correctly, check whether Slow Ramp-Up is turned on in Instantly and adjust it there; it's an Instantly campaign setting, not something we control or that reflects a problem with your mailbox.
Other errors while connecting or importing
Other errors during setup almost always come down to one of the following. Check these in order:
Your mailboxes aren't active yet. If any accounts still show as pending or inactive, wait until they're fully Active and then retry the export. Accounts that aren't active can't be imported.
The Instantly credentials need a second look. Re-open Settings → Sequencers → Instantly → Configure and confirm you entered your Instantly login email and password (not an API key), and that 2FA is turned off on that Instantly account.
Your subscription needs attention. If your plan has expired, or you have more mailboxes or domains than your current plan covers, the affected accounts drop out of the active state and won't connect. Renewing or updating your subscription so it covers all of your mailboxes and domains restores them to active, and the import will then go through.
A note on Google Workspace mailboxes
Google Workspace mailboxes connect using the sender credentials we provide for each account, rather than IMAP. When you export your accounts to Instantly, the correct connection details are included automatically, so you don't need to look them up yourself.
Still stuck after checking the steps above? Contact us with the mailboxes that are failing to connect and we'll review your specific case and get them working.