As of now, Proton Mail does not support IMAP, so you will have to connect such an inbox either:

1) Using an SMTP-only option. Please note that SMTP-only cannot be used in the free trial, nor with the Basic, PRO, or MAX plan. You will have to purchase a dedicated Send-only plan.

2) You can use ProtonMail Bridge, a software application provided by ProtonMail, to enable IMAP access for your ProtonMail account. More information can be found on this website. By using ProtonMail Bridge you will be able to connect your inbox using the trial, Basic, PRO, or MAX plan.

1) In your browser, sign in to your Proton Mail account and select Settings → All settings → Proton Mail → IMAP/SMTP → SMTP tokens.

2) Click Generate token.

3) Enter the following details to create a new SMTP token:

– Token name: Select a name so you’ll remember where this token is used, such as the service or device name. If you need to allow multiple third-party clients to use SMTP, generate a different token for each client so you can manage their permissions independently. This increases security as you can remove a client’s SMTP permission by deleting its token without affecting the other clients.

– Email address: Select one of your active custom domain addresses to pair with your token. Copy this email address and use it as the username in the third-party client. If you have multiple addresses, generate a different token for each address.

4) Click Generate.

5) Enter your Proton Mail Account password.

Your SMTP username and SMTP token (password) will be generated. You can now enter them as the username and password of your Proton Account in Warmup Inbox.

SMTP username: your generated username

SMTP password: your SMTP token

SMTP host: smtp.protonmail.ch

Port: 587

SSL/TLS: OFF

To learn more about this connection you can visit Proton Mail Help Section.