Why are emails not sent with SMTP?

Question

symfony

On the site page there is a form for sending a letter to the client. The email is sent by PHP using an SMTP service via Symfony Swift Mailer. Server smtp.gmail.com, port 465, SSL option enabled. Some of the letters do not reach the recipients. It was not possible to determine the conditions under which
this error occurs. What can be wrong?

Answer 1

you need to start with the log files on the server, as well as register the return address and check it.
If the letters do not reach, there are 3 options:
1) they are filtered at the end point (for example, by a spam filter or antivirus)
2) they are wrapped along the way
3) they are simply not sent.
What will help to find out the reason:
1) analyze which letters do not reach and to which domains (actually a rather rotten option)
2) analyze the response letters. Often, an undelivered letter is returned back with a description of the reason for blocking
3) the reason for "not sending" should be in your logs.
Start by looking for the send logs for all "missed" emails.