Hi @robinwin , Welcome to the community! If you are sending e-mails to a large number of recipients (Exp: More that 50 recipients) please enable the SMTP authentication on the e-mail client settings to avoid getting banned by the server. Our mail server will only allow a maximum of 300 email recipient per batch, if it exceeds you will experience SMPT authentication restriction. Note: You also need to consider the file size of the email attachment POP3 and IMAP can only handle up to 20MB attached file. If you require to send an 470 to 500 emails. We recommend to do it by batch. 300 recipient per batch and It is best if you give 15mins interval before you send another batch of 300. Note: Ensure that SMTP Authentication enabled and you're sending from a TPG connection (connected via TPG service).   Let us know should you require further assistance. Cheers! ... View more