There are two reasons:
(1) Some mailboxes require your web login to set up the IMAP/POP3/SMTP service. If you do not have it, you cannot log in to your account using MeSince.
(2) Since MeSince is a new email client software, many free email service providers cannot identify and do not allow a strange email client software to log in to the mailbox. You should receive a notification email and say like “Less secure app” that may be a security risk. You just need to approve and consent to use MeSince to login to your mailbox.