Autoresponder landing in subscriber's SPAM

Hi All, I set up an autoresponder in my Wordpress website so that when a new guest enters their email they will get a thank you and an attachment of a short story. They choose either a .docx or a .pdf and the file will open for them. However, most if not all of these 'thank you with attachment' emails go into my new subscriber's SPAM folder. I get people asking me why I didn't send them the file almost every time. I have tested this and in the past it wasn't a problem, but the last year or so more and more often it goes into a person's SPAM file. How can I stop this from happening? I've tried using different email servers: one is Mail.com and the other is the service that came with my Wordpress, hosted by hostgator, which is ' I have tried to add a gmail account as the sender to prevent my email looking like SPAM to reciepient servers, but I can't seem to find the navigation to do this (I must have done it once or twice, obviously). Please advise. I do warm my new subscribers to check ther SPAM but this should be an exception, not the norm, right? Thank you!