I haven't had any issues with either SMTP or IMAP on Fastmail. I've even used their IMAP server by hand with no issues. Here is the IMAP capability list (which shows protocol extensions supported) before logging in:

* CAPABILITY IMAP4 IMAP4rev1 LITERAL+ ENABLE UIDPLUS SASL-IR NAMESPACE CONDSTORE SORT LIST-EXTENDED QRESYNC MOVE SPECIAL-USE CREATE-SPECIAL-USE IDLE AUTH=PLAIN

and after:

* CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE ACL RIGHTS=kxten QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SEARCH=FUZZY SORT SORT=MODSEQ SORT=DISPLAY SORT=UID THREAD=ORDEREDSUBJECT THREAD=REFERENCES THREAD=REFS ANNOTATE-EXPERIMENT-1 METADATA LIST-EXTENDED LIST-STATUS LIST-MYRIGHTS LIST-METADATA WITHIN QRESYNC SCAN XLIST XMOVE MOVE SPECIAL-USE CREATE-SPECIAL-USE DIGEST=SHA1 X-REPLICATION STATUS=SIZE OBJECTID SAVEDATE X-CREATEDMODSEQ XAPPLEPUSHSERVICE LOGINDISABLED XCONVERSATIONS COMPRESS=DEFLATE X-QUOTA=STORAGE X-QUOTA=MESSAGE X-QUOTA=X-ANNOTATION-STORAGE X-QUOTA=X-NUM-FOLDERS IDLE

They only have the TLS ports open; the plain text services are disabled (no need for STARTTLS since that's insecure from MITM).

They allow having a password different from the account password specifically for these services.

That initial set of capabilities comes from the NGINX proxy and is hard-coded as the set of things which seem to be needed to make clients interoperate well. The second response (yeah, I know it's long!) is from the Cyrus IMAPd server on your backend. I see you captured that right now though, because X-CREATEDMODSEQ has only been there about a week. It's probably going to be renamed as GLOBALMODSEQ or something eventually... check out https://mailarchive.ietf.org/arch/browse/extra/ for all the gory details.

I usually recommend Fastmail to people who ask me for recommendations and aren't comfortable running their own mail server.

What benefits does Fastmail have over ProtonMail?