We have that same issue! Weirdly, never had this issue before, but we had no updates.
Never needed the sendmail provider package, added it anyways, didn’t help unfortunately.
We’re using strapi v4.15.0 with sendmail 4.15.4.
When using Strapi no-reply@strapi.io as default sender and test-sending to our own company email, the email is successfully sent according to the success notification, but never delivered. When using gmail as recipient of test email we receive the can not connect to any SMTP server error.
Any help with this? Our goal is merely to send an email for inviting users and sending forgot-password-emails.