Broken IPv6 / IPv4 configuration, or Gmail brokenness?

[Celejar <celejar@gmail.com>]

Hello,

My Debian Sid system, including its networking system, has been
working fine for a while. Recently, Gmail has not been working properly
on the system: sending (via SMTP with SSL) times out, and receiving
(via POP3 and IMAP) takes abnormally long. Today I finally made some
efforts to understand what's going on. Here's what I know /
understand / have been able to establish:

1) My ISP provides me with only IPv4, not IPv6.

2) Trying to send email through Gmail using "smtp.gmail.com", via
Sylpheed and Swaks, times out without getting anywhere. For example:

$ swaks -t celejar@gmail.com -s smtp.gmail.com:587 -tls -a LOGIN
Username: celejar
Password: ********
=== Trying smtp.gmail.com:587...
*** Error connecting to smtp.gmail.com:587:
*** 	IO::Socket::INET6: connect: timeout

3) Forcing the use of IPv4 fixes the timeout problem. In particular,
both of the following work (in the second command, the IPv4 address is
the one returned by "dig" and "host" for "smtp.gmail.com"):

$ swaks -t celejar@gmail.com -4 -s smtp.gmail.com:587 -tls -a LOGIN
$ swaks -t celejar@gmail.com -s 142.251.163.109:587 -tls -a LOGIN

So, what's going on here? I suppose I don't really need IPv6 on my
system, but everything I've read says that it should be left enabled and
not disabled. Is something misconfigured on my system, is this a Gmail
problem, or am I missing something else?

-- 
Celejar