UIDs and GIDs are two different types of identifiers for users and groups in Linux. They are not related to each other, and they do not affect each other's availability or functionality. A user can have the same UID as another user's GID, or vice versa, without any problem. The only restriction is that UIDs and GIDs must be unique within their own domain, i.e., no two users can have the same UID, and no two groups can have the same GID. Having the same UID as a GID does not imply any special relationship between the user and the group, nor does it grant any extra permissions or access rights. The user and the group are still treated as separate entities by the system123 Reference: 1: Linux sysadmin basics: User account management with UIDs and GIDs 2: How to (Correctly) Change the UID and GID of a user/group in Linux 3: Linux File Permission: uid vs gid - CBT Nuggets

Postfix, Sendmail, and Exim are three of the most commonly used Mail Transfer Agent (MTA) applications on Linux systems. An MTA is a software that transfers and routes electronic mail messages from one computer to another using the Simple Mail Transfer Protocol (SMTP). An MTA receives messages from another MTA or from a Mail User Agent (MUA), which is a computer application that end users use to access or send emails. An MTA can also query the MX records of the recipient's domain to find the destination mail server and forward the message accordingly. An MTA can also perform other functions such as filtering, encryption, authentication, and bounce handling.
Postfix is a cross-platform, popular MTA that was designed and developed by Wietse Zweitze Venema for his mail server while working at the IBM research department. It was primarily developed as an alternative to well-known and popular Sendmail MTA. Postfix runs on Linux, Mac OSX, Solaris, and several other Unix-like operating systems. It borrows a lot of Sendmail properties on the outside, but it has a totally and comprehensively distinct internal operation. Additionally, it bids to be fast in performance with easy configurations and secure operation mechanism1.
Sendmail, now known as Proofpoint (after Proofpoint, Inc acquired Sendmail, Inc), is by far the most popular and one of the oldest MTA on the Linux server platform. Sendmail has a lot of limitations though, in comparison to modern MTAs. Because of its complicated configuration steps and demands, and weak security mechanisms, many new MTAs have come up as alternatives to Sendmail, but importantly, it offers everything to do with mail on a network1.
Exim is a free MTA developed for Unix-like operating systems such as Linux, Mac OSX, Solaris, and many more. Exim offers a great level of flexibility in routing mail on a network, with outstanding mechanisms and facilities for incoming mail monitoring. Its notable features include among others: no support for POP and IMAP protocols, supports protocols such as RFC 2821 SMTP and RFC 2033 LMTP email message transport, configurations include access control lists, content scanning, encryption, routing controls among others1.
Procmail is not an MTA, but a mail processing utility that can be used to filter, sort, and deliver incoming mail. It can be invoked by an MTA or run as a standalone program. Procmail can process mail based on various criteria such as sender, subject, header, body, size, date, and more. It can also execute external programs, forward mail to another address, or write mail to a file.
SMTPd is not an MTA, but a generic name for a daemon (a background process) that implements the SMTP protocol. A daemon is a program that runs continuously and performs certain tasks at predefined times or in response to certain events. An SMTP daemon listens for incoming SMTP connections from other MTAs or MUAs and handles the mail transfer accordingly. SMTPd can also refer to a specific SMTP daemon that is part of the OpenSMTPD project, which is a free implementation of the SMTP protocol for Unix systems. Reference:
7 Best Mail Transfer Agents (MTA's) for Linux
Mail Transfer Agent (MTA) Explained | Mailtrap Blog
What is a Message Transfer Agent (MTA)? - Definition from Techopedia
Mail Transfer Agent (MTA) - Glossary of Email Terms | Mailgun
[Procmail - Wikipedia]
[SMTP daemon - Wikipedia]