as a plug-in for WordPress with an inquiry form Contact Form 7 This plugin is well known for its ability to create a site using it, but here’s what you can do about it.

fail to get an automatic reply e-mail detected by spam

If you do not receive the specified e-mail address even though you have contacted us, there are many cases where mail is in a folder of junk mailbox.
This is because people are suspected of spoofing someone else's e-mail address and have been judged to be junk mail. WP Mail SMTP It can be avoided by introducing a plug-in that sends mail in SMTP such as "A" and proving it is an email address owned by the person.
And since Contact Form 7 doesn’t store the message you sent somewhere, I recommend sending it with SMTP in the sense of backing up.

not receive any e-mail

Even though the transmission completion screen appears on Contact Form 7, e-mail may not be included in the junk mail folder.
Apparently this happens when an email address is automatically converted from the same domain (http://example.com) to a mail address of the same domain (@example.com). WP Mail SMTP I was able to avoid it by introducing a plugin that sends e-mail in SMTP such as “Support” and so on.

Conclusion

When using Contact Form 7: WP Mail SMTP It is also safe to introduce it at the same time.