OVH DNS settings for fastmail.com. They can be pasted "as text" in OVH panel to replace all current settings. You need to replace "example.fr" with your domain.

ovh_dns_settings_for_fastmail.txt

$TTL 3600
@	IN SOA dns200.anycast.me. tech.ovh.net. (2022082311 86400 3600 3600000 60)
                          IN NS     ns200.anycast.me.
                          IN NS     dns200.anycast.me.

                          IN A      66.111.4.53
                          IN A      66.111.4.54
                          IN MX     10 in1-smtp.messagingengine.com.
                          IN MX     20 in2-smtp.messagingengine.com.
                          IN TXT    "v=spf1 include:spf.messagingengine.com ~all"

*                         IN A      66.111.4.53
*                         IN A      66.111.4.54
*                         IN MX     10 in1-smtp.messagingengine.com.
*                         IN MX     20 in2-smtp.messagingengine.com.
*                         IN TXT    "v=spf1 include:spf.messagingengine.com ~all"

mail                      IN A      66.111.4.147
mail                      IN A      66.111.4.148
mail                      IN MX     10 in1-smtp.messagingengine.com.
mail                      IN MX     20 in2-smtp.messagingengine.com.
mail                      IN TXT    "v=spf1 include:spf.messagingengine.com ~all"

fm1._domainkey            IN CNAME  fm1.example.fr.dkim.fmhosted.com.
fm2._domainkey            IN CNAME  fm2.example.fr.dkim.fmhosted.com.
fm3._domainkey            IN CNAME  fm3.example.fr.dkim.fmhosted.com.
_dmarc                    IN TXT    "v=DMARC1;p=reject;pct=100;rua=mailto:[email protected];ruf=mailto:[email protected]"


_submission._tcp          IN SRV    0 1 587 smtp.fastmail.com.
_imap._tcp                IN SRV    0 0 0 .
_imaps._tcp               IN SRV    0 1 993 imap.fastmail.com.
_pop3._tcp                IN SRV    0 0 0 .
_pop3s._tcp               IN SRV    10 1 995 pop.fastmail.com.
_jmap._tcp                IN SRV    0 1 443 api.fastmail.com.

_carddav._tcp             IN SRV    0 0 0 .
_carddavs._tcp            IN SRV    0 1 443 carddav.fastmail.com.
_caldav._tcp              IN SRV    0 0 0 .
_caldavs._tcp             IN SRV    0 1 443 caldav.fastmail.com.

Sources that have helped me with this:

https://simonandrews.ca/articles/how-to-set-up-spf-dkim-dmarc

https://stackoverflow.com/questions/64495457/all-all-and-all-in-dns-spf-configuration

https://www.fastmail.help/hc/en-us/articles/360060591153-Manual-DNS-configuration