Unable to connect to SMTP server to send messages from Turnpike

[#1]

See extract from the connection log:

Sat, 9 Sep 2023 10:02:10 SMTP[C22] Starting to send mail to relay.plus.net
Sat, 9 Sep 2023 10:02:31 SMTP[C22] <- 421 Cannot connect to SMTP server 212.159.8.107 (212.159.8.107:25), connect error 10060
Sat, 9 Sep 2023 10:02:31 Mail connection to relay.plus.net closed, 0 messages transmitted
Sat, 9 Sep 2023 10:02:31 SMTP command rejected while talking to relay.plus.net:  421 Cannot connect to SMTP server 212.159.8.107 (212.159.8.107:25), connect error 10060
Sat, 9 Sep 2023 10:02:50 SMTP[C23] Starting to send mail to relay.plus.net
Sat, 9 Sep 2023 10:03:11 SMTP[C23] <- 421 Cannot connect to SMTP server 212.159.8.107 (212.159.8.107:25), connect error 10060

[#2]

@chrispb smtp error 421 is

Your message was temporarily deferred by the recipient server. This is usually a result of too many connections in a short timeframe or too many messages.

If you're off-network i.e not on a Plusnet connection then the sending limit is quite low.

Wait for a while and it should allow sending again.