Why is a TLS/SSL certificate needed when sending emails, does it affect the deliverability of emails?

Question

Exim

У меня несколько вопросов, на которые ищу ответ.
1. Для чего необходим TLS/SSL сертификат при рассылке писем?
2. Письмо подписывается SSL сертификатом только при рассылке методом SMTP или же при всех видах (mail, sendmail, smtp) ?
3. Влияет ли он на доставляемость писем?
4. Стоит Exim, пользуюсь PHP mailer(последняя версия). Необходимо ли дополнительно указывать на сертификат в настройках PHP mailer, или же он подгружается из настроек exim?

Answer 1

Why do I need a TLS/SSL certificate when sending emails?

To encrypt the transport layer between the sending server and the receiving one. During the exchange, "comrade major" will not be able to read the contents of the letter (but the metadata - from whom, to whom, subject, etc. - will be able to)
Don't get confused. The letter is signed with a certificate, if such is specified in the settings of the mail client - Outlook, TB. This requires the issuance of personal certificates and separate configuration. Using a certificate at the transport layer does not sign anything. It encrypts.
JFYI: No SMTP distribution . There is an SMTP protocol that is used when sending emails. There is no "any other method". Sendmail is an SMTP server, and mail is generally a console mail client :)
3. Does it affect email deliverability?
Not

Answer 2

The letter is not signed with a certificate, the certificate is used to establish a secure connection between mail servers (STARTTLS). Directly, the presence or absence of STARTTLS does not affect delivery, but GMail, for example, can show users an icon if an email is received over an insecure connection. You do not need to specify anything in the mailer settings, because the certificate is used between MTA (exim in your case).

Answer 3

Point 3. Deliverability is affected by the presence of DKIM, SPF and a decent PTR