Emails are not being sent to the assigned user on a ticket. We are using a plugin (Redmine Up Helpdesk) to pull in emails from our server which then post replies on tickets.

If the ticket is updated through a logged in user, the email notification sends to the assigned user.

If the ticket is update through an incoming email, not email is sent.

I checked the production logs and it looks like it's failing with the ActionMailer

[ActiveJob] [ActionMailer::DeliveryJob] [6000be6e-3ef4-450b-a7ec-2c685f14cd4b] Email delivery error: 354 Start mail input; end with .

This was working fine when we upgraded to 4.0.3 but two weeks later this started happening

We do have 2 other plugins,
Redmine Banner and RedmineUp Contacts

I am making a ticket with RedmineUp to see if this caused by their plugins as well.

Any information to what is causing this be helpful