So, I run an online service where people can sign up using their email address. It's as basic as it can get. Put in your address, receive email with confirmation link.
For that I've set up mutt on Debian 11 which I'm using to send a HTML-EMail with an attachment.
For some reason, mail providers like googlemail and outlook filter those mails out. Sometimes they appear in peoples inbox, sometimes in spam and sometimes they don't get them at all. My googlemail UI shows me that it "couldn't verifiy if this email came from DOMAIN or from a spammer".
Does anyone here know what I can do to resolve this problem?
Any advice is appreciated.