Same experience with Mailgun. The "free" tier servers do not have good reputations. As you can imagine, being free, they are used by spammers. I found it imposs

Same experience with Mailgun. The "free" tier servers do not have good reputations. As you can imagine, being free, they are used by spammers. I found it impossible to send to yahoo.com recipients. Gmail, interestingly, did accept and deliver almost all messages.