Stephen,
Can you please provide a bit more detail and your setup. To me the issue maybe DNS record related, but I am not sure. I am guessing mail.stackmail.com certificate has expired or is self-signed? But I am not sure how to check this.
Do you use any DNS caching on your computer? Can you try a different computer?
Is insilicochemistry.net your web site, and do you also have one or more MX records (insilicochemistry.net, and mail.insilicochemistry.net) in your web site's DNS that point to mail.stackmail.com ?
Below is some DNS name checking that I did, the results confuse me.
I am guessing that your web site insilicochemistry.net is hosted on scluster24.stablehost.com and your email server is hosted on mail.stackmail.com?
If so, what happens if in your Epyrus email client, you use mail.stackmail.com and not mail.insilicochemistry.net ?
Epyrus is not packaged with Debian, hence I expect that you downloaded it and installed it?
http://www.epyrus.org/
https://mxtoolbox.com/SuperTool.aspx?action=mx%3ainsilicochemistry.net&run=toolpage
Pref Hostname IP Address
10 mx.stackmail.com 185.151.28.67
$ dig mx mail.insilicochemistry.net +short
mail.stackmail.com.
gkirkham@debkde05:~$ dig mx insilicochemistry.net +short
$ dig -x 213.109.149.117 +short
scluster24.stablehost.com.
$ nslookup insilicochemistry.net
Non-authoritative answer:
Name: insilicochemistry.net
Address: 213.109.149.117
$ nslookup insilicochemistry.net -type=mx
Non-authoritative answer:
Name: insilicochemistry.net
Address: 213.109.149.117
$ nslookup 213.109.149.117
117.149.109.213.in-addr.arpa name = scluster24.stablehost.com.
$ nslookup 213.109.149.117 -type=mx
Non-authoritative answer:
117.149.109.213.in-addr.arpa name = scluster24.stablehost.com.
$ nslookup mail.insilicochemistry.net
Non-authoritative answer:
mail.insilicochemistry.net canonical name = mail.stackmail.com.
Name: mail.stackmail.com
Address: 185.151.28.68
Name: mail.stackmail.com
Address: 185.151.28.84
Name: mail.stackmail.com
Address: 185.151.28.85
$ nslookup mail.insilicochemistry.net -type=mx
Non-authoritative answer:
mail.insilicochemistry.net canonical name = mail.stackmail.com.
Name: mail.stackmail.com
Address: 185.151.28.68
Name: mail.stackmail.com
Address: 185.151.28.84
Name: mail.stackmail.com
Address: 185.151.28.85
https://www.stablehost.com/about-us.php
$ nslookup scluster24.stablehost.com -type=mx
Non-authoritative answer:
Name: scluster24.stablehost.com
Address: 213.109.149.116
$ nslookup 213.109.149.116
116.149.109.213.in-addr.arpa name = scluster24.stablehost.com.
$ ping 185.151.28.68
PING 185.151.28.68 (185.151.28.68) 56(84) bytes of data.
64 bytes from 185.151.28.68: icmp_seq=1 ttl=45 time=296 ms
64 bytes from 185.151.28.68: icmp_seq=2 ttl=45 time=296 ms
^C
--- 185.151.28.68 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 296.197/296.311/296.425/0.114 ms
$ nslookup 185.151.28.68
68.28.151.185.in-addr.arpa name = smtp.stackmail.com.
$ nslookup 185.151.28.85
85.28.151.185.in-addr.arpa name = 185-151-28-85.ptr4.stackcp.net.
https://mxtoolbox.com/SuperTool.aspx?action=mx%3amail.insilicochemistry.net&run=toolpage
Type Domain Name Canonical Name TTL
CNAME mail.insilicochemistry.net mail.stackmail.com 60 min Blacklist Check SMTP Test
Test Result
Status Problem DMARC Record Published No DMARC Record found Information More Info
Status Warning DMARC Policy Not Enabled DMARC Quarantine/Reject policy not enabled Information More Info
Status Ok DNS Record Published DNS Record found
https://mxtoolbox.com/SuperTool.aspx?action=smtp%3amail.insilicochemistry.net&run=toolpage
https://mxtoolbox.com/emailhealth/mail.insilicochemistry.net/
5 Problems
Category Hos